Introduction

Carbon monoxide (CO) is an achromatic, odorless and tasteless gas that can be poisonous for humans due to serious threats for the environment including acid rain, ozone depletion and secondary pollutants production [1, 2]. Any burning materials and fuels containing carbon are considered to be the main sources of CO. Wide range of flammable carbon monoxide [3] along with the release of CO mixture in chemical accidents [4] motivated researchers to find effective ways for capturing CO from defective burned atmosphere which is a great improvement in health issues [5]. Therefore, in recent decades, the development of cost-effective technologies for capturing CO has attracted tremendous attentions [6,7,8].

Various types of porous materials have been applied for CO capture, and it is still full of challenges. Surface properties of porous adsorbents such as well thickness, surface area, and pore-size distribution make their applications increased significantly [9,10,11,12,13,14,15] including nanoporous materials such as metal–organic frameworks (MOFs) [16, 17], mesoporous alumina (MA) [18, 19], and mesoporous silica (MS) [20, 21] which are known as an alternative to other commercial adsorbents such as zeolite and activated carbon [22, 23].

Because of the uniform structure of the porous nanomaterials MS, MOFs, and MA and their high surface areas, the adsorption capacity of these adsorbents is significant [24]. Accordingly, high adsorption capacity of alumina-based substances is due to the uniform pore size, interlinked channels, and the united porous structures [25, 26]. The usage of alumina-based materials as a catalyst in purification processes (e.g., hydrotreatment, hydrocracking, and modification), along with their role as adsorbents, in particular, for toxic gases removal, is widely known [27]. Among seven types of alumina phases known as “transition alumina” [28], due to the impurity and defects of their crystal lattice, the stable phase belongs to α-alumina while γ-alumina has less stability [29, 30]. γ-Phase nano-Al2O3 with large surface area, pore-volume, and high catalytic activity, as one of the most important and newest ceramic materials, is the best candidate for capturing gas molecules, as the same as mesoporous alumina (MA) [31, 32]. Therefore, herein, the γ-phase nano-Al2O3 has been used as the base substance. Remarkable progress has been carried out in the past few years for γ-Al2O3 synthesis [33].

In addition, the development of palladium (Pd) and palladium (II) nitrate nanoparticles is an important issue due to their applications as catalysis, water denitrification and CO gas adsorption because of their remarkable properties [34] along with their superior performances [35, 36] due to their tiny uniform pores [37]. The smaller particle size of Pd clusters on the surface of Al2O3 favors a higher CO adsorption and enhances the adsorption capacity of the adsorbent.

Moreover, zeolite (Ze) nanoparticles are considered as crystalline aluminosilicates (or silicates) with two-dimensional regular arrangements of pores. Zeolite nanoparticles have unique properties such as high surface areas, exchangeable cations, molecular sieving [38].

Hence, as the major aim of the present study, Pd(NO3)2, zeolite and Al2O3 were loaded on glass substrates by roll coating technique to improve the ability of adsorbents for CO adsorption and increase the range of reactions between CO gas molecules and adsorbents surface. In addition, the adsorption capacity and efficiency of Al2O3/Pd(NO3)2/zeolite composite films were calculated which are equal to 111.16 (mg g−1) and 97%, respectively, and also adsorption kinetic mechanism was studied by pseudo-first-order, pseudo-second-order, and intra-particle diffusion models. The morphology has been probed by field emission scanning electron microscopy (FESEM), and structural properties were also explored by X-ray diffraction (XRD).

Experimental details

Adsorption of CO

Here, CO (99,999%) was used as a target gas. The schematic of designed and made experimental setup for measuring CO adsorption is given in Fig. 1. As can be seen from Fig. 1, the set up consists of a capsule as a source of CO gas, the compartment (20 cm length and 7 cm internal diameter), in which an adsorbent is putted, and a gas analyzer apparatus (KIMO KIGAZ 210) for evaluating CO gas concentration. In this study, temperature and pressure were held constant at 32 °C (at room temperature) and 1.5 bar, respectively. The inlet CO gas concentration was 150 mg L−1, and the saturation level of CO gas concentration was 5 mg L−1.

Fig. 1
figure 1

Schematic of the experimental setup used for CO gas adsorption testing on Al2O3/Pd(NO3)2/zeolite composite film consists of CO gas capsule (purity = 99.999%), an adsorbent placement, a gas analyzer device (KIMO KIGAZ 210)

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Materials

Zeolite nanoparticles (Al2O34SiO2H2O, purity: > 99%) and alumina nanoparticles (Al2O3, gamma, purity: > 99.9%) were purchased from Nanoshel chemicals. Palladium nitrate (Pd(NO3)2) and 1-methyl-2-pyrrolidone were bought from Sigma-Aldrich and Merck chemicals, respectively. All received chemicals were used without extra purification.

Preparation of adsorbent

Al2O3/Pd(NO3)2/zeolite composite films have been deposited on glass substrates by roll coating method. Four glass substrates (2 cm × 8 cm) were washed three times by disinfectant materials such as acetone, ethanol, and deionized water in an ultrasonic device and dried at room temperature. The process of preparation of samples was including 1 gr Al2O3: 1 gr zeolite: 1 gr Pd(NO3)2 mixed in a small container, and then, 10 CC of 1-methyl-2-pyrrolidone was slowly added dropwise into it. Final suspensions were stirred for about 1 h and then used for coating on glasses. The prepared coated substrates were desiccated at room temperature for 1 day. Finally, four Al2O3/Pd(NO3)2/zeolite-coated substrates were attached together to make a hollow cubic container as shown in Fig. 2, to study CO adsorption on Al2O3/Pd(NO3)2/zeolite composite films. In this case, it behaves as a tunnel in which gas molecules are channeled and trapped readily. Thus, the rate of interaction between gas molecules and adsorbents is enhanced which will affect the adsorption capacity and efficiency.

Fig. 2
figure 2

Schematic of fabricated Al2O3/Pd(NO3)2/zeolite adsorbent in the form of cubic

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Characterization

The structural and morphological properties of Al2O3/Pd(NO3)2/zeolite composite film were characterized. The crystalline structure of the composite film was analyzed by X-ray diffraction (XRD, STOE STADI MP). The visualization of topography and morphology of prepared samples were analyzed by field emission scanning electron microscope (FESEM, MIRA3 TESCAN), while chemical and elemental contents of the sample were measured by energy-dispersive X-ray spectroscopy (EDX) analysis system attached with scanning electron microscope. The thickness of the prepared sample was determined by profilometer analysis (RAGA). The gas analyzer device (KIMO KIGAZ 210) was applied for CO gas adsorption test.

Results and discussion

XRD

Figure 3 shows the XRD patterns of Al2O3/Pd(NO3)2/zeolite adsorbent. XRD patterns were recorded using an X-ray diffractometer with Cu Kα source (λ = 1.5405 Å) and a scan step size of 0.01°. The scanning range (2θ) was recorded between 10° and 90°. The XRD peaks of the composite film were considered at 10.98°, 13.05°, 17.23°, 22.31°, 25°, 29°, 37°, 44.50°, 46.22°, 51.73°, 57.45°, which were assigned to the crystalline preferred orientation of 220, 110, 121, 200, 040, 125, 110, 323, 202, 420, 116, respectively (Table 1). As can be seen in the XRD patterns, the adsorbent shows three major peaks (at 2θ = 22.31°, 29°, 44.50°) due to the presence of zeolite and palladium II nitrate.

Fig. 3
figure 3

XRD patterns of a the Al2O3/Pd(NO3)2/zeolite composite film, b reference γ-Al2O3 (JCPDS 00-011-0661), Pd(NO3)2 (JCPDS 00-001-0398) and zeolite (JCPDS 00-042-0018 and 01-087-2276) nanoparticles

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Table 1 XRD peaks and crystalline areas of the Al2O3/Pd(NO3)2/zeolite composite film
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DXRD was used to find the size of nanoparticles from Debye–Scherrer’s equation and XRD [39,40,41] (1):

$$ D_{\text{XRD}} = \frac{K\lambda }{{\beta {\text{Cos}}\left( \theta \right)}} $$
(1)

where K is known as the shape factor or Scherrer’s constant that varies in the range 0.89 < K < 1, and usually is 0.9 (assuming that the particles have spherical shape), λ is the X-ray wavelength (1.54178 Å), β is full width at half maximum (FWHM) of the diffraction peak and θ is the diffraction angle. The particle diameters of the samples are summarized in Table 2.

Table 2 Particle diameters of Al2O3, Pd(NO3)2, zeolite
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FESEM

In order to determine surface morphology, particle size and distribution of the prepared adsorbent, field emission scanning electron microscope (FESEM) analysis was employed. FESEM images of Al2O3/Pd(NO3)2/zeolite sample at 200 nm scale of magnification before and after CO gas adsorption are presented in Fig. 4.

Fig. 4
figure 4

FESEM images of the made adsorbent with Al2O3/Pd(NO3)2/zeolite nanoparticles a before and b after CO gas adsorption at 200 nm scales of magnification

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The development of united porous structures with regular interlinked channels is observable in the adsorbent after adsorption. As it is obvious, the FESEM image after adsorption represents more homogeneous dispersion and well particle size repartition in respect to the virgin film. These properties are responsible for the high surface area and therefore very high adsorption for CO. The average size of particles is equal to 37.41 nm. The result from the profilometer analysis determined that the thickness of the Al2O3/Pd(NO3)2/zeolite composite film is 6966.7 nm.

EDX

Energy-dispersive X-ray spectroscopy (EDX) was carried out to determine the percentage of elemental content. EDX pattern in Fig. 5 confirms the presence of the ingredients which were utilized as adsorbents. The spectrum of the droplets shows a notable increase at the spectral position of Al EDX peak. The increase of aluminum peak is due to the use of alumina nanoparticles and zeolite (aluminosilicates) nanoparticles in this study. Table 3 reveals the weight and atomic percentages of ingredients extracted from EDX patterns of the adsorbent including Al, O, Pd as well as N and Si at 32.57, 55.16, 1.57, 4.10, 7.59 wt% for each element. In addition, Ca peak corresponded to glass substrates [42].

Fig. 5
figure 5

EDX result of the Al2O3/Pd(NO3)2/zeolite composite film

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Table 3 Atomic and weight percentage values of the Al2O3/Pd(NO3)2/zeolite composite film by statistical analysis of EDX spectrum
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Adsorption of CO

The variation of concentration of adsorbed CO versus time for Al2O3/Pd(NO3)2/zeolite adsorbent at constant temperature and pressure is shown in Fig. 6. As can be seen, the concentration of adsorbed CO gas (mg L−1) is increased with increasing time (s). However, the decrease of slope indicates that the adsorption speed became lower since it reaches saturation region.

Fig. 6
figure 6

The diagram of the concentration of adsorbed CO gas (mg L−1) as a function of time (s)

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The adsorption efficiency (R%) which specifies the performance of adsorbent for CO adsorption, adsorption capacity at time t (qt, mg g−1), and adsorption capacity (qe, mg g−1) which evaluates the concentration of adsorbed CO gas through the adsorbent at equilibrium were calculated by using Eqs. (2), (3), and (4), respectively [43]:

$$ R = \frac{{\left( {C_{0} - C_{e} } \right)}}{{C_{0} }} \times 100 $$
(2)
$$ q_{t} = \frac{{\left( {C_{0} - C_{t} } \right) \times V}}{M} $$
(3)
$$ q_{e} = \frac{{\left( {C_{0} - C_{e} } \right) \times V}}{M} $$
(4)

where C0 (mg L−1) is the inlet concentration, Ct (mg L−1) is concentration of adsorbed CO gas at time t, Ce (mg L−1) is concentration of adsorbed CO gas at equilibrium, V(L) is the volume of the chamber, and M(g) is the weight of the adsorbent.

The variation of adsorption efficiency (R%) versus time for CO gas adsorption on Al2O3/Pd(NO3)2/zeolite adsorbent is shown in Fig. 7. As can be seen, the adsorption efficiency is increased with increasing time (s). The maximum percentage of CO adsorption is equal to 97% which occurred at 216 (s), and then, a saturation region was developed indicating that vacant sites in Al2O3/Pd(NO3)/zeolite are saturating with CO molecules while time is passing [44].

Fig. 7
figure 7

The adsorption efficiency (%) of CO gas as a function of time (s) curve

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Figure 8 shows the variation of adsorption efficiency versus concentration of adsorbed CO gas which is a single, smooth, and linear curve. It demonstrates that efficiency is increased by increasing the concentration of adsorbed CO gas, and it is continuous until it leads to a saturation region.

Fig. 8
figure 8

The effect of outlet CO gas concentration (mg L−1) on adsorption efficiency (%)

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Figure 9 shows adsorption capacity (qt, mg g−1) versus time (s) diagram. The results revealed obvious enhancing in the adsorption capacity of Al2O3/Pd(NO3)2/zeolite adsorbent for CO adsorption with the increase of time (s). The maximum adsorption capacity is 111.16 mg g−1. Also, at this point, the saturation point was started and defined as “adsorption capacity at equilibrium time.” This means that adsorption sites were saturated with CO molecules, and there are no other sites to attach to CO molecules with increasing time [45].

Fig. 9
figure 9

The relation between time (s) and the uptake capacity (mg g−1)

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Table 4 gives adsorption capacity, adsorption efficiency, inlet gas concentration, and concentration at the saturation level of CO gas adsorption results on Al2O3/Pd(NO3)2/zeolite film. According to Table 5, by comparing with other published articles, the results indicate that adsorption capacity of Al2O3/Pd(NO3)2/zeolite composite film is higher than Ze, Si, AC, Pd/Si and Pd/AC adsorbents, and lower than MA and Pd/MA adsorbents [20].

Table 4 Parameters of adsorption capacity and adsorption efficiency for CO gas on Al2O3/Pd(NO3)2/zeolite composite film at room temperature (32 °C)
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Table 5 Comparison of adsorption capacity parameters for CO gas on Al2O3/Pd(NO3)2/zeolite composite film and various adsorbents at room temperature (32 °C)
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Kinetic study

The pseudo-first-order, pseudo-second-order and intra-particle diffusion models are applied to analyze the kinetic study of CO gas adsorption on Al2O3/Pd(NO3)2/zeolite composite films.

Pseudo-first- order kinetic

Pseudo-first-order model is applicable to study adsorption process during the initial stage [46,47,48]. The pseudo-first-order model (Lagergen, 1898) [49] is defined as Eq. (5):

$$ { \log }\left( {q_{e} - q_{t} } \right) = { \log }q_{e} - \frac{{K_{\text{ad}} }}{2.303}t $$
(5)

By determining the intercept (\( { \log }\,q_{e} \)) and slope (\( \frac{{K_{\text{ad}} }}{2.303} \)) of a linear plot of \( { \log }\left( {q_{e} - q_{t} } \right) \) vs. t, calculated equilibrium adsorption density (qe) and pseudo-first-order constant (Kad) can be calculated [46]. Figure 10 shows a linear plot of \( { \log }\left( {q_{e} - q_{t} } \right) \) versus time. According to Table 6, the experimental value of qe,exp is not agreement with the theoretical value of qe,cal. The regression coefficient (R2) is 0.86. Therefore, the low values of R2 and negative slope of \( { \log }\left( {q_{e} - q_{t} } \right) \) vs. t indicate the inadequacy of the pseudo-first-order model to describe interaction among Al2O3/Pd(NO3)2/zeolite molecules [50].

Fig. 10
figure 10

Pseudo-first-order kinetic model for CO adsorption by Al2O3/Pd(NO3)2/zeolite composite films

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Table 6 Comparison of pseudo-first-order, pseudo-second-order and intra-particle diffusion models parameters
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Pseudo-second -order kinetic

In order to investigate the influence of chemical potential of adsorbent which is sensitive to temperature, time and gas concentration on adsorption process, the pseudo-second-order model is applied [45, 48, 51]. The equation related to pseudo-second order is given as [46, 52]:

$$ \frac{t}{{q_{t} }} = \frac{1}{{k_{2} q_{e}^{2} }} + \frac{t}{{q_{e} }} $$
(6)

The linear \( \frac{t}{{q_{t} }} \) versus t plot gives intercept \( \frac{1}{{k_{2} q_{e}^{2} }} \) and slope \( \frac{1}{{q_{e} }} \) to determine pseudo-second-order constant (K2, g mg−1 s−1) and theoretical qe,cal calculated value [46]. Figure 11 indicates a linear plot of \( \frac{t}{{q_{t} }} \) versus t for Al2O3/Pd(NO3)2/zeolite composite films which remained stable until 80 s and then increased with passing time.

Fig. 11
figure 11

Pseudo-second-order kinetic model for CO adsorption by Al2O3/Pd(NO3)2/zeolite composite films

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Kinetic parameters of Al2O3/Pd(NO3)2/zeolite adsorbent are given in Table 6. The experimental and theoretical values of qe are not in agreement with each other. Also, the smaller value of K2 than K2q2e (initial rate constant) indicates the fast CO adsorption process during the initial period of time, and then, it was getting slower with time [47, 53]. The value of regression coefficient (R2) for this model is close to unity that it is applicable to explain CO adsorption by Al2O3/Pd(NO3)2/zeolite adsorbent [46, 54].

Intra-particle diffusion kinetic model

The intra-particle diffusion kinetic model is a common model to characterize diffusion mechanism of CO molecules and Al2O3/Pd(NO3)2/zeolite composite films. The intra-particle model is defied by Eq. (7) [46, 55]:

$$ q_{t} = K_{\text{diff}} t^{1/2} + C $$
(7)

where Kdiff (mg g−1 sec1/2) is intra-particle diffusion constant which can be obtained by the slope of the qt vs. t1/2 plot [46] (Fig. 12).

Fig. 12
figure 12

Intra-particle diffusion kinetic model for CO adsorption by Al2O3/Pd(NO3)2/zeolite composite films

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According to the published articles [44, 56,57,58], the linear plot of qt vs. t1/2 for Al2O3/Pd(NO3)2/zeolite adsorbents through the whole time process shows that adsorption is affected by only one proceeding. Also, intra-particle diffusion is a rate-controlling step because the plot passes through the origin [56, 57]. Table 6 gives value of intra-particle diffusion constant.

Conclusion

The present study aimed to improve CO adsorption through adsorbent and increase the range of interactions between CO gas molecules and adsorbent. Therefore, the roll coating technique was applied for the preparation of Al2O3/Pd(NO3)2/zeolite composite films through loading Pd(NO3)2, zeolite and Al2O3 nanoparticles on glass substrates. While the inlet CO gas concentration was 150 mg L−1, adsorbed CO gas concentration was calculated as a function of reaction time. The concentration of adsorbed CO gas was increased by passing time until 216th seconds and then it reached a saturation level of 5 mg L−1 due to the increase of contact surface area of adsorbent particles with CO gas molecules. Moreover, adsorption efficiency (R%) which showed the performance of adsorbent for CO adsorption was increased by increasing time and increasing concentration of adsorbed CO gas until it reached the saturation level with the maximum value of 97%. Uptake capacity (qt) was also defined to evaluate the concentration of adsorbed CO through the adsorbent and increased with the increase of time and remained nearly constant with slight changes by increasing time with the maximum value of 111.16 mg g−1. Kinetic study was investigated by pseudo-first-order, pseudo-second-order, and intra-particle diffusion models. CO adsorption process was the best fit by pseudo-second-order model with the high value of R2 (0.98). CO adsorption occurred through one step according to intra-particle diffusion model. Also, intra-particle diffusion is rate-controlling step because the plot passes through the origin. Elemental content of Al, O, Pd, Si, and N as well as Ca which was referred to glass substrates was observed in EDX analysis while the crystalline structure of Al2O3/Pd(NO3)2/zeolite composite films with their particles diameters and FWHM was characterized through XRD patterns. The interconnected channels in the structures of Al2O3/Pd(NO3)2/zeolite surfaces in FESEM images with united porous structures are responsible for the efficient capture of CO gas molecules. Moreover, homogeneous dispersion and well particle size repartition of Al2O3/Pd(NO3)2/zeolite adsorbent with the average size of 37/41 nm were obtained.