Design of a meso-structured Pd/NiO catalyst for highly efficient low temperature CO oxidation under ambient conditions

Abstract

A meso-structured Pd/NiO catalyst was successfully fabricated through a controlled pyrolysis and in situ reduction protocol. The resulting material possessed a relatively high surface area and highly dispersed palladium species. It showed much higher catalytic activity and stability for CO oxidation under ambient conditions. Complete CO conversion could be achieved at as low as −20 °C, when 1.2 vol% H₂O was introduced into the feed gas. The catalyst exhibited no detectable deactivation even after 100 hours of reaction. The extraordinary catalytic activity and durability were attributed to the promotion of the water molecules and the synergetic effect between the Pd nanoparticles and meso-structured NiO support.

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Introduction

Carbon monoxide, one of the most common and widely distributed air pollutants, and is harmful to the environment and human health. It originates mainly from incomplete combustion of carbonaceous materials, such as exhausts of internal combustion engines and industrial processes, etc. There is a general agreement that catalytic oxidation is the most effective way to eliminate this diluted air pollutant, in which the key issue is the availability of high performance catalysts under ambient conditions.¹ To date, many catalytic systems for low-temperature CO oxidation have been well developed, including supported noble metals and transition metal oxides.^2–4 However, most of them can only be used under dry conditions.^5–8 Generally, moisture inevitably exists especially for practical application of low-temperature CO oxidation processes and plays completely different roles in the presence/absence of noble metals. It could be a devastatingly poisonous species for transition metal oxides or a promoter for noble metals.^2,4,9–16 Thus, attention for low-temperature CO oxidation under ambient conditions has been mainly focused on supported noble metal catalysts.

The most commonly used supported noble metal catalyst is nano-sized gold supported on metal oxide. It is generally accepted that the gold nanoparticles dispersed on the reducible oxides catalysts are extraordinarily active for CO oxidation and have much stronger durability under moisture conditions.^9–18 But there are still some unsolved deficiencies when in practical use, such as deactivation in storage and under indoor light, over-high sensitiveness to halogen-including compound et al.^19–22 The alternatives are platinum group metal based catalysts, which have much stronger anti-poison properties, and are also promising catalysts for CO oxidation especially for that supported on FeO_x and Fe(OH)_x. However, these reported catalysts always need relatively high temperature for complete CO conversion under moisture conditions.^22–34 Low-temperature CO oxidation catalyst for practical application under ambient conditions is still expected and in challenge.

It is well known that synergy between active component and support play a key role in determining the catalytic performance.³⁵ Benefited from the various oxidation states, charge-variable transition metal oxides possess excellent performance in oxygen storage and release which are also critical factors for low-temperature CO oxidation. When appropriate charge-variable transition metal oxide was selected and combined with Pd nanoparticles, significantly enhanced activity could be expected. In this paper, the mesoporous nickel oxide was chosen as support for Pd catalyst. The palladium component was transferred into the porous structure through wet impregnation and the noble metal nanoparticles were in situ produced using hydrazine hydrate as reductant. The palladium nanoparticles could be homogeneously deposited in/on the NiO supports, and thus possessed the much enhanced catalytic activities. The CO oxidation reaction of the materials under different conditions and the reaction mechanism were also discussed in detail.

Experimental section

Materials preparation

Mesoporous NiO support was prepared following a reported approach of oxalate decomposition.³⁶ Briefly, 10 mmol Ni(NO₃)₂·6H₂O was dissolved into 100 ml aqueous solution, then 10 ml 1 M sodium oxalate was slowly added under vigorous stirring. The precipitate was filtered, washed with deionized water and dried at 60 °C. The mesoporous NiO material was finally obtained after calcinated at 300 °C for 2 h. The heating rate during calcination should be controlled to be as low as 1 °C min⁻¹.

A series of mesoporous NiO supported Pd catalysts were synthesized through an improved wet impregnation procedure. In a typical synthesis, 0.5 g mesoporous NiO material was added in 25 ml aqueous solution containing calculated content of Na₂PdCl₄, After stirred for 2 h, 0.1 M hydrazine hydrate (NH₂·NH₂·H₂O) was slowly added and stirred for 12 h successively. The resulting solid mass was collected through centrifugation, washed with water and dried at 60 °C for 24 h.

Characterization

Powder X-ray diffraction (XRD) patterns were recorded on a Bruker D8 Focus powder diffract meter with graphite mono chromatized Cu Kα radiation (λ = 0.15405 nm) operated at 40 kV. Thermo gravimetric analysis (TG-DSC) were carried out at a heating rate of 10 °C min⁻¹ from ambient temperature to 700 °C with an air flow rate of 20 ml min⁻¹. Nitrogen adsorption and desorption isotherms were measured on a Micromeritics ASAP 2020 M analyzer at liquid nitrogen temperature (77 K). Prior to the measurements, the samples were degassed at 423 K in vacuum for 6 h. The specific surface area and pore size distribution were calculated using the Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH) methods, respectively. Transmission electron microscopy (TEM) observations were performed on a field emission JEM-2100 (JEOL) electron microscope operated at 300 kV equipped with a Gatan-666 electron energy loss spectrometer and energy dispersive X-ray spectrometer. XPS (X-ray photoelectron spectroscopy) signals were collected on a VG Micro MK II instrument using monochromatic Al Kα X-ray at 1486.6 eV operated at 200 W. All the elemental binding energies were referenced to the C (1s) line situated at 284.6 eV. H₂ temperature-programmed reduction (H₂-TPR) analysis was performed by using a Micromeritics Chemisorb 2750 apparatus. For each analysis, accurate amounts of calcined sample (50 mg) were purged in a flow of pure argon at 200 °C for 30 min to remove traces water (heating rate 10 °C min⁻¹). After cooling to room temperature, H₂-TPR experiments were performed using a 10 vol% H₂–Ar mixture at a flow rate of 25 ml min⁻¹. The sample was heated from ambient temperature to 500 °C at heating rate of 10 °C min⁻¹ and H₂ consumption was detected by a thermal conductivity detector (TCD).

CO oxidation test

The catalytic test for CO oxidation was carried out in fixed-bed quartz tubular reactor (i.d. = 6 mm) containing 200 mg of catalyst samples without any pretreatment. A standard reaction gas contained 1.0 vol% CO balanced with air was introduced into the reactor using mass-flow controllers at a flow rate of 50 ml min⁻¹, corresponding to a space velocity of 15 000 ml h⁻¹ g_cat⁻¹. The conversion of CO was measured by online gas chromatograph (GC) under steady-state condition. The activity of the catalyst in the presence of moisture condition was tested by passing the feed gas at a flow rate of 50 ml min⁻¹ through a water vapor saturator. Changing the temperature of saturator can adjust the water concentration in the feed gas.

In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) analysis

Infrared spectra were recorded by a FTIR spectrometer (Nicolet iS10) equipped with a MCT detector. The sample cell was fitted with ZnSe windows. The sample was exposed to the corresponding reaction stream (60 ml min⁻¹). The DRIFTS spectra obtained at 25 °C with a solution of 4 cm⁻¹ and 50 scans. Typical gas mixture was 0.5 vol% CO, 20.0 vol% O₂ balanced with He. Water vapor was carried into the gas mixture by a bubbler in a water bath at room temperature. After a designated amount of time, the CO flow was switched to a humid stream containing 1.5 vol% water.

Results and discussion

The mesoporous NiO support was obtained through direct pyrolysis of nickel oxalate and the palladium component was transferred into the porous structure through wet impregnation and in situ reduction strategy. The actual contents of Pd in the meso-structured Pd/NiO materials were determined by the ICP-AES technique to be 1.0, 2.8, 5.3 and 7.3 wt%, respectively. The result shows that almost all of the palladium species in the aqueous solution have been deposited into/on the mesoporous NiO support.

X-ray powder diffraction

For supported catalysts, the crystalline structure and surface chemical state of support and active components often play crucial roles in the performance of the catalysis. The crystalline structure of the nickel oxalate precursor and resulting Pd/NiO materials were first characterized by X-ray diffraction (XRD) analysis. The as-synthesized nickel oxalate is well crystallized containing two coordinated water corresponding well with the PDF card no. 10-0742 (Fig. 1A). After calcination, the nickel oxalate hydrate was totally decomposed and converted to nickel oxide (Fig. 1B). A series sharp and symmetrical Bragg diffraction peak can be found in wide angle region which demonstrates its well crystallized feature. All the peaks in the pattern can be well assigned as the characteristic reflection planes of NiO (PDF card no. 73-1523). When loaded with palladium, the diffraction peaks do not have any visible change. The (111) diffraction peaks of face-centered cubic structure palladium are almost unobservable at 2θ = 40° even at a high Pd loading of 7.3 wt%. The extremely weak characteristic XRD peak of metallic Pd implies the highly dispersed amorphous or small palladium particles.

Fig. 1 XRD patterns of NiC₂O₄·2H₂O precursor (A) and Pd/NiO catalysts with different Pd loading contents (B): 0 wt% (a), 1.0 wt% (b), 2.8 wt% (c), 5.3 wt% (d) and 7.3 wt% (e).

X-ray photoelectron spectroscopy

To get the details of the chemical state of palladium and surface NiO, the materials were then examined with X-ray photoelectron spectroscopy (XPS), as shown in Fig. 2. For NiO support, the Ni 2p region only comprises the typical NiO feature (853.0 eV, 854.6 eV). After loaded with Pd, the typical peak for hydroxylated NiO emerged (855.2 eV). Therefore, the Ni 2p spectra of Pd/NiO could be decomposed as the signal of pure NiO and hydroxylated NiO. With increase of Pd loading content, the intensity of hydroxylated NiO increases. This phenomenon could be assigned to the presence of significant surface hydroxylation.^37–42 In Pd 3d region, the typically binding energy of Pd⁰ at 335.3 and 340.5 eV for 3d_5/3 and 3d_3/2 can be observed. The slight higher energy at 337.4 and 342.5 eV can be assigned to the Pd²⁺. However, the peaks are very weak indicating the existence of a small amount Pd²⁺. Element analysis indicated that the Pd⁰ in the catalysts are all above 85%. This clearly confirms that most of the Pd²⁺ ions on the surface have been reduced to Pd⁰ by hydrazine hydrate. With the increase of Pd loading amount, the intensity of Pd spectra increases.

Fig. 2 XPS spectra of Ni 2p (A) and Pd 3d (B) for the NiO support (a) and Pd/NiO catalysts with different Pd loading contents: 1.0 wt% (b), 2.8 wt% (c), 5.3 wt% (d) and 7.3 wt% (e).

Texture properties

Scanning electron microscopic (SEM) analysis clearly indicates that NiO support inherit the morphology of its corresponding oxalate precursor (Fig. S2, ESI†). Except that some fragments and broad cracks, limited size shrinkage can be found after thermal treatment. It can be inferred that there must be significant porosity left within the NiO particles. Further transmission electron microscopy (TEM) observations (Fig. 3) prove our speculation. Pores are randomly but homogeneously distributed within the whole particle, which are produced by a great quantity of gases slowly released from the oxalate precursor leaving behind numerous voids during thermal decomposition process.³⁶ After impregnation, the noble metal ion species, PdCl₄²⁻, was transferred into the porous structure of nickle oxide. The following in situ reduction results in Pd/NiO material with Pd nanoparticles being homogeneously dispersed in/on the mesoporous support. No large noble metal particle can be found in the TEM images even Pd loading content reached to 7.3 wt%. In addition, the selected area electron diffraction (SAED) patterns from the same area are also presented in the TEM images. The well-defined diffraction rings for NiO can be found in all the samples. However, the relative faint diffraction ring of palladium indicates its much lower crystallinity. These results are in good agreement with above XRD analysis. In addition to the crystalline structure, the morphology and distribution of supported palladium species have been examined by high resolution transmission electron microscopy (HRTEM) (Fig. S3, ESI†). It clearly shows that all the catalysts are composed of crystallized NiO and Pd nanoparticles. Nanocrystals with different lattice d-spacing values can be found all over the images. Except for those with the d-spacing 2.2 Å assignable to Pd crystallite, all of them can be identified as NiO nanoparticles. The particle size distributions of palladium species show that the highly dispersed Pd nanoparticles on the NiO support are mostly smaller than 5 nm (Fig. S5, ESI†). The further energy dispersive X-ray spectroscopy (EDS) mapping images confirm the enrichment of palladium and its high dispersion on the NiO support. The impregnation and in situ reduction method have been proved to be effective in fabricating noble metal loaded NiO composite catalysts. The noble metal nanoparticles could be homogeneously deposited into/on the polycrystalline NiO support (Fig. S4, ESI†).

Fig. 3 TEM images of NiO support (A) and Pd/NiO catalysts with different Pd loading contents: 1.0 wt% (B), 2.8 wt% (C), 5.3 wt% (D) and 7.3 wt% (E) (the HRTEM images were obtained from the circular region).

The texture properties of the resulting materials were further investigated by measuring adsorption–desorption isotherms of nitrogen at 77 K, as shown in Fig. 4. In all the cases, the typical Langmuir IV isotherms suggest the mesoporous structure and the appearance of H3 hysteresis loops indicate the formation of slit-like meso-pore. The average pore size is about 2.1 nm and the specific surface area is as high as 272 m² g⁻¹. When loaded with palladium, the specific surface area, pore size and pore volume of the Pd/NiO materials only show slight decrease (Table S1, ESI†). These results demonstrate that palladium nanoparticles have been well dispersed into the pore channels and/or on the surface of the support. The most of the pore channels remain open, which is important to allow reactant molecules to diffuse into the mesostructure and access the catalyst nanoparticles.

Fig. 4 N₂ adsorption–desorption isotherms of NiO support (A) and Pd/NiO catalysts with different Pd loading contents: 1.0 wt% (B), 2.8 wt% (C), 5.3 wt% (D) and 7.3 wt% (E).

H₂-TPR analysis

Fig. 5 shows the reduction profiles of the catalysts, and for comparison, the H₂ temperature programmed reduction (H₂-TPR) profile of the parent NiO is also included. The reduction process of Ni²⁺ to Ni⁰ takes place at 350 °C, which is proved by the XRD analysis (Fig. S6, ESI†). After loaded with palladium, the reduction peak shifts to relative lower temperature region. Such a shift is related with spillover effect involving either hydrogen activated on the metal phase or mobile lattice oxygen induced by intimate metal–support interactions.¹² The significant low temperature reduction feature strongly suggests that the nickel oxide substrate has been ‘activated’ to a large extent in the Pd/NiO catalyst by Pd loading.

Fig. 5 H₂-TPR of NiO support (a) and Pd/NiO catalysts with different Pd loading contents: 1.0 wt% (b), 2.8 wt% (c), 5.3 wt% (d) and 7.3 wt% (e).

Catalytic activities for CO oxidation

The catalytic performance of NiO and corresponding Pd loaded samples for CO oxidation under dry conditions are shown in Fig. 6. The NiO support does not show any obvious catalytic activity at reaction temperatures lower than 100 °C. The Pd loaded material, 5.3 wt% Pd/NiO, shows the highest catalytic activity. The complete CO conversion temperature is as low as 50 °C. Although this temperature is somewhat higher than expected, it still exhibits excellent catalytic performance of CO oxidation compared with the NiO supported Au catalysts.² However, when water molecules were introduced into the feed gas, the catalytic performance of Pd/NiO catalyst is quite different. It shows a dramatic enhancement (Fig. 6B). The 5.3 wt% Pd/NiO material still demonstrates the highest catalytic activity. The 100% conversion temperature decreased from 50 °C to −20 °C, when 1.2% water was introduced into the feed gas. Continuously increasing the Pd content, the catalytic performance remains almost unchanged, which clearly indicates that not only the content of active component (palladium), the dispersion of palladium nanoparticles and the synergetic effect between palladium and NiO may also play important roles in CO oxidation process.

Fig. 6 The catalytic performances of NiO support and Pd/NiO catalysts with different Pd loading contents under dry (A) and moisture conditions (B).

Catalytic stability for CO oxidation

The catalytic stability of the 5.3 wt% Pd loaded catalyst was also tested under the moisture conditions. Surprisingly, the 1.0 vol% CO was completely converted and the complete conversion keeps unchanged even after 100 h reaction at 25 °C (Fig. 7). This may be attributed to the very reactive absorbed intermediate, COOH, which is formed from the H₂O molecular reaction with CO.^23,43 The presence of water molecules could lead to the decreased activation energy and thus promote the CO oxidation. Moreover, to gain insight into the intrinsic activities of supported Pd catalysts under different conditions, TOFs normalized by the number of the surface noble atoms from differential reaction results were compared (Table S2, ESI†). No matter under the dry or moisture condition, the TOFs of the 5.3 wt% Pd loaded catalyst all give the maximum values (14.14 × 10⁻⁴ s⁻¹ under dry condition and 31.69 × 10⁻⁴ s⁻¹ under moisture condition, respectively) among all samples synthesized. After reaction, the catalyst was characterized by XRD, TEM and N₂ adsorption–desorption measurement, no structure collapse or phase segregation has been observed, indicating the stable state of the catalyst (Fig. S8–S10, ESI†).

Fig. 7 The catalytic stability 5.3 wt% Pd loaded catalyst under moisture conditions at 25 °C.

Effect of moisture

In order to explore the effect of H₂O in CO oxidation process, different amounts of H₂O were first introduced into the feed gas, as shown in Fig. 8. The catalytic activity increases with the increased water amount from 0.4 vol% to 1.2 vol%. Further increasing the water content does not produce additional enhancement of the catalytic activity. Under the relatively higher humidity condition, H₂O may capillary-condensed in the mesopores of the NiO support, consequently preventing the reactants from accessing to catalyst surface and reduced the catalytic activity.

Fig. 8 The activity of 5.3 wt% Pd loaded catalyst under different moisture conditions.

In addition, in situ DRIFTS technology was also employed to explore the difference of surface reaction of CO on 5.3 wt% Pd/NiO catalyst under different humidity conditions. According to the literatures, the wide bands at 1360 and 1470 cm⁻¹ are ascribed to υ_s(OCO) and δ(OH) of [COOH]_s, respectively.^44–46 The strong band at 1600–1700 cm⁻¹ can be assigned to the OH groups,^47,48 whereas, the weak band at 1810 cm⁻¹ is related to bonded CO on metallic Pd.^49,50 Fig. 9A shows the change in band intensity of surface species over Pd/NiO catalyst under dry conditions. Upon exposure to CO in dry stream, the bands at 1650 cm⁻¹ attributed to surface –OH group were observed. This phenomenon is corresponding well with the above XPS analysis. Interestingly, the band intensity gradually decreased with the contact time, which means the surface –OH group participation in the CO oxidation process. In contrast, under the moisture conditions (Fig. 9B), due to the adequate supply from water, the intensity of –OH group remains relatively constant during the whole process. Besides, compared with that under dry stream, Pd–CO band appear and become stable more rapidly. The presence of water could not only be in favor of the formation of carbonate species (COOH), but also promote the adsorption of CO on the surface of metallic Pd. All these clearly indicate the positive effect of water in CO oxidation on the surface of Pd/NiO catalysts.

Fig. 9 In situ DRIFT spectra of 5.3 wt% Pd/NiO under 0.5 vol% CO–20.0 vol% O₂–He (A) and 0.5 vol% CO–20.0 vol% O₂–1.5 vol% H₂O–He (B) at 25 °C.

Conclusions

The meso-structured Pd/NiO catalysts have been successfully prepared by a combined nickel oxalate decomposition and in situ reduction protocol. The resulting composite material possessed relatively high specific surface area and highly dispersed Pd nanoparticles. Among all samples prepared, 5.3 wt% Pd loaded catalyst showed the highest reaction activity in the low-temperature CO oxidation reaction, the complete conversion could be achieved at as low as 50 °C under dry conditions, much lower than that of NiO supported Au catalysts. More importantly, this material showed greatly enhanced catalytic activity and stability under moisture conditions. The complete CO conversion temperature can be decreased from 50 °C to −20 °C when 1.2 vol% H₂O was introduced into the feed gas. The catalytic activity remained un-deteriorated even after 100 hours of reaction. The in situ DRIFTS analysis demonstrates the promotion of H₂O in CO oxidation process over Pd/NiO catalyst.

Acknowledgements

This study was supported by National Basic Research Program of China 2013CB933201.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra05591a

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