4.1. Adsorption Isotherms
The adsorption isotherms of resin II and
n-C
7 asphaltenes at 25 °C for the CeNi0.89Pd1.1 and CeO
2 nanoparticles together with the SLE model fitting are shown in
Figure 2. A high affinity can be observed between the
n-C
7 asphaltenes and the different nanoparticles used. The shape of the adsorption isotherms in the two systems is Type Ib according to the International Union of Pure and Applied Chemistry (IUPAC) [
44]. This type of isotherm indicates a strong affinity between the adsorbate and the adsorbent. This property is always higher for the CeNi0.89Pd1.1 than for the CeO
2 nanoparticles, indicating an increase in the adsorptive capacity of the nanomaterial with the addition of the transition element oxides (TEO) on its surface [
45] through the formation of coordinated bonds between the heteroatoms and the functional groups of the transition elements [
46].
Regarding the adsorption isotherms of resin II onto CeNi0.89Pd1.1 nanoparticles, a high affinity is observed over the range of concentrations evaluated. This behavior, according to the International Union of Pure and Applied Chemistry (IUPAC), corresponds to a Type I isotherm [
44]. This result is due to the interactions formed by the functional groups of NiO and PdO on the nanoparticle surface with the active bonds and heteroatoms in the chemical structures of the resins [
47]. Regarding the adsorption of resin II on the non-functionalized support, a type II isotherm is obtained [
44]. When the concentration of resin II increases, molecules with a larger size have reduced diffusion through the material surface, leading to a high self-association around the nanoparticles’ active sites [
48]. This indicates that, although the resin–resin interactions are low in the bulk phase, once adsorbed on the nanoparticle surface, self-association occurs through coordinated bonds and bonds between carbon-based molecules and heteroatoms (C–C, C–N, C–O) because of their polar character [
49,
50]. In this sense, for both resins and asphaltenes, the adsorption is higher for CeNi089Pd1.1 nanoparticles than for the support without TEOs. Because of the presence of transition elements, the adsorption of polar molecules on the nanoparticles is controlled to a high degree by the Lewis acidity [
51], which favors the adsorption on the functionalized nanoparticles compared to the support. The nanoparticle that showed the best performance by adsorbing resin II and
n-C
7 asphaltenes was CeNi0.89Pd1.1. Therefore, the competitive adsorption between
n-C
7 asphaltenes and resin II for different R:A ratio was evaluated with this sample, and the results are shown in
Figure 3a,b together with the SLE model fitting.
Figure 3a shows that the adsorption affinity between
n-C
7 asphaltenes and CeNi0.89Pd1.1 nanoparticles decreases as the amount of resin II in the system increases. For the R:A ratios of 2:8 and 1:1, the adsorption isotherms are also Type I, and for the 8:2 system, the isotherm obtained behaves as type III according to the IUPAC [
44]. The change in the adsorbate–adsorbent affinity may be related to an interruption or change in the colloidal structure of
n-C
7 asphaltenes because of the presence of resins that prevents asphaltene–asphaltene interactions, thus avoiding self-association [
52,
53]. Also, because of the high selectivity that CeNi0.89Pd1.1 nanoparticles present for resin II, competitive adsorption between the two polar fractions may be generated. In this way, there is a reduction in the active sites on the surface of the nanoparticles occupied by resin. These results are consistent with those published by Lozano et al. [
36] and Franco et al. [
26], where the adsorption of asphaltenes on nanoparticles of different chemical nature is affected to a small extent by the presence of resin I in the system.
Figure 3b shows the adsorption isotherms for resin II in the presence of
n-C
7 asphaltenes at different R:A ratios. In general, it can be seen that the adsorption isotherms keep their type I behavior for the ratios 8:2 and 1:1. However, for the 2:8 system, a high presence of
n-C
7 asphaltenes generates a change in the type of isotherm to Type III. In both cases, it is indicated that the adsorption of resin II is also influenced by the presence of
n-C
7 asphaltenes. The adsorption of resins II over the nanoparticles increases as the amount of
n-C
7 asphaltenes in the system decreases, following the order 2:8 < 1:1 < 8:2. The interactions formed between the functional groups of the metal oxides and the heteroatoms in the molecular structure of resins II are stronger than the resin–resin and resin–asphaltene interactions because of the presence of acidic and basic sites that the TEO and CeO
2 generate [
54]. The multilayer behavior of the 2:8 ratio could be due to the resin–asphaltene interactions being more significant than the resin–resin interaction [
55].
Table 3 shows the
,
, y, and
parameters of the SLE model for the adsorption of asphaltenes and resin II in different R:A ratios over the functionalized and CeO
2 nanoparticles. According to the
H parameter, there is greater affinity for CeNi0.89Pd1.1 material with the resin II and
n-C
7 asphaltenes as individual components. For the adsorption isotherms of
n-C
7 asphaltene in the presence of resin II, the
parameter increased as the amount of resin II in the system increased because of the high selectivity that CeNi0.89Pd1.1 nanoparticles presented for resin II. In this way, it appears that the preference of
n-C
7 asphaltenes to be present in the adsorbed phase is reduced by the presence of resin II. Concerning the degree of self-association of
n-C
7 asphaltenes on the surface of the nanoparticles, the
parameter follows the R:A trend of 2:8 < 1:1 < 8:2, which indicates that the asphaltene–asphaltene interactions are reduced because of the presence of stronger asphaltene–resin interactions [
55,
56]. Also, the value of
for the adsorption of resin II decreases as the amount of
n-C
7 asphaltene increases, and the
parameter behaves similarly to the self-association of
n-C
7 asphaltenes. This is also due to the fact that resin–resin interactions are affected by asphaltene–resin interactions [
57].
4.2. Prediction of the Adsorbed Amount of n-C7 Asphaltenes
For a better understanding of the influence of resin II on the adsorption phenomenon of n-C7 asphaltenes over the CeNi0.89Pd1.1 nanoparticles, the adsorbed amount of asphaltenes was predicted as individual systems for the different R:A ratios employed. If the behavior of n-C7 asphaltenes were affected by the amount of resin II in the systems, there would be a difference between the predicted and the experimental adsorbed amounts. On the other hand, if the resin had no effect on the asphaltene adsorption, the adsorption of the latter would depend only on its concentration and the occupation of the active sites of the nanoparticle by resin II. For the prediction of the adsorbed amount of n-C7 asphaltenes (), it was assumed that for a fixed R:A system, the amount adsorbed would correspond to a fraction of the amount of individual asphaltenes adsorbed. That is, for a 1:1 system, the amount of adsorbed n-C7 asphaltenes corresponds to 50% of the mass fraction initially adsorbed. A linear plot versus that represents a good prediction would imply a slope of = 1 and an intercept of = 0 associated with = 1.0, indicating that asphaltene adsorption depends mainly on the amount of asphaltenes present in the system.
Figure 4 shows the prediction plot of the adsorbed amount of
n-C
7 asphaltenes in the CeNi0.89Pd1.1 nanoparticles in the absence and presence of resin II. In addition,
Table 4 shows the values of the slope and the intercept of the equation associated with the linear fit and the respective errors. According to
Figure 4 and the data in
Table 4, a very accurate prediction was observed for the adsorbed amount of
n-C
7 asphaltenes on CeNi0.89Pd1.1 nanoparticles in the presence of resin II for the R:A systems evaluated. The values of the slopes in all cases were close to 1, and the intercepts were close to the origin with
> 0.98. These results indicate that the adsorption of
n-C
7 asphaltenes is controlled mainly by the concentration of these in the system and by the affinity that the nanoparticle presents for resin II. These results are in agreement with those reported by Franco et al. [
26].
4.3. Thermogravimetric Experiments
The nanoparticles evaluated in this work were tested in an N
2 atmosphere saturated with H
2O
(g) for the catalytic steam gasification of
n-C
7 asphaltenes and resin II.
Figure 5 shows the rate of mass loss for
n-C
7 asphaltenes and resins II in panels a and b, respectively. It can be observed from
Figure 5 that for virgin resin II and
n-C
7 asphaltenes, i.e., before adsorption onto nanoparticles, the decomposition occurs at 420 and 450 °C, respectively. Nevertheless, when they are adsorbed on the nanoparticles, their decomposition occurs at lower temperatures.
Functionalized and non-functionalized nanoparticles reduce the main decomposition peak from 400 to 220 and 270 °C for resin II and from 450 to 220 and 370 °C for n-C7 asphaltenes, respectively. However, the heavy oil fractions do not decompose completely at these temperatures, and therefore, the conversion continues at higher temperatures. This result suggests the presence of high, medium, and low molecular weight sizes that vary from alkyl chains that decompose at low temperatures (<250 °C) to polycyclic aromatic hydrocarbons (PAH), whose decomposition is achieved at average temperatures of around 450 °C.
Besides, the functionalized nanoparticle manages to reduce the resin II decomposition temperature to 220 °C, which inhibits the development of additional reactions of resin II after its initial cracking, showing a single peak of decomposition [
16]. The catalytic effect was improved with the addition of oxides of transition elements on the CeO
2 surface for
n-C
7 asphaltene and resin II decomposition. The good performance of the functionalized nanoparticles is due to the ability of the support to interact with the nanocrystals to produce and promote cracking and isomerization reactions [
58]. The Ce–Ni interactions are able to promote the production of the water–gas shift reaction through vacancies of oxygen anions provided by the CeO
2 [
20]. Meanwhile, the interactions of the support with noble elements like Pd [
22] promote the production of hydrogen and oxygen which controls the stability of highly polar molecules. Hence, further experiments under isothermal conditions were performed with the functionalized nanoparticles and for different R:A ratios.
Figure 6 shows the isothermal conversions at 220 °C for different R:A ratios of 8:2, 1:1, and 2:8 and for the individual components. The conversion of resin II at any time is always greater than the conversion of asphaltenes. In addition, the conversion of the R:A systems decreases as the amount of
n-C
7 asphaltenes in the mixture increases because of the high presence of heteroatoms and metals as well as the high refractory behavior of
n-C
7 asphaltenes.
4.4. Effective Activation Energy
For the effective activation energy estimation, a plot of
vs.
was obtained using the conversion of each fraction at three different temperatures. The value of the activation energy was obtained from the slope. The isothermal conversions for resin II and
n-C
7 asphaltenes in the presence of nanoparticles were made at 210, 220, and 230 °C, and for the fraction in the absence of nanoparticles, the employed temperatures were 360, 370, and 380 °C. These curves are shown in
Figures S2–S4 of the
Supplementary Materials.
Figure 7 shows the activation energy values for the resin II and
n-C
7 asphaltene decomposition in the presence and absence of the functionalized nanoparticles and the CeO
2 support. There is a significant decrease in the activation energy values for resin II adsorbed on the different nanoparticles, with the CeNi0.89Pd1.1 system being the one that generates the biggest decrease in
. The presence of oxides on the surface of the nanoparticle generates a change in the decomposition mechanism of the heavy oil fractions, and their performance depends, to a considerable extent, on the chemical nature of the nanoparticle. In addition, the activation energy values are lower for the decomposition of resin II than for
n-C
7 asphaltenes.
The activation energy values for the R:A systems in the presence and absence of CeNi0.89Pd1.1 nanoparticles were calculated by following the same procedure and the results are shown in
Figure 8. The required activation energy increases as the amount of asphaltenes in the system increases, both in the presence and absence of the CeNi0.89Pd1.1 nanoparticles. Nevertheless, the nanoparticles reduce the activation energy values necessary to carry out the decomposition reactions of the systems to 21.51 ± 1.05 kJ∙mol
−1 independently of the amount of asphaltenes in the system.
Cerium oxide nanoparticles have the ability to regenerate their Ce
4+ atoms through a redox mechanism, maintaining the catalytic power of the support. This is achieved by carrying out processes of adsorption of reagents (H
2O and heavy oil fractions), decomposition and chemical change of adsorbed molecules, desorption of reaction products between the support and reagents (CO
2, CH
4, LHD, CO, among others), and the interactions between the products and the active sites of the nanocatalyst. Here, the chemical reactions between the hydrocarbons Equation (9) and carbon monoxide Equation (10) with CeO
2 to produce Ce
3+ ions are shown, where
is the oxygen vacancy produced from the redox cycle Ce
4+/Ce
3+:
In this way, the reduction of Ce
4+ to Ce
3+ leads to the production of water vapor molecules and CO
2. Then, through a redox cycle, the initial oxidation state is recovered and the decomposition of asphaltenes and resins continues. However, the high performance of this catalyst is also associated with the active phases formed by the TEO on the surface of the support. The resulting species –O and –OH from the dissociative adsorption of water by the lower valence state of cerium may be transferred to nickel and palladium and react with surface carbonaceous species. Besides, through the movement of oxygen vacancies formed by the change in the oxidation state of the CeO
2±δ species and the destabilization of the same, the reagents are transferred to the active sites of the transition element oxides, as schematized in
Figure 9.
4.5. Dynamic Tests of Oil Recovery
The oil recovery curve for the dispersed injection of CeNi0.89Pd1.1-based nanofluid in the steam stream to assist the steam injection technology is shown in
Figure 10. In addition, the saturation states of both fluids, water and oil, are shown in
Table 5. For these tests, the effective permeabilities of water (kw) and oil (ko) were estimated to be 3744 and 2264 mD, respectively, which is in agreement with the properties of an HO reservoir. After the injection of 7 PVWE, an oil recovery of 51% was obtained because of the transfer of heat to the rock and fluids, promoting its thermal expansion. In addition, other related mechanisms are the possible gravitational segregation of fluids, the volatilization of the lightest hydrocarbons and the disaggregation of the viscoelastic network of the oil. After the dispersed injection of the nanofluid in the steam, an increment of 25% in the oil recovered was obtained. This is, to a great extent, due to the small size of the liquid droplets as they are carried by steam stream to achieve higher penetration. Finally, after soaking the porous medium for 12 h, steam was injected again, and an ultimate oil recovery of 93% was reached.
The good performance obtained in the test carried out through nanofluid injection dispersed by the steam stream compared with the batch injection reported in previous studies [
9,
18] is related to the rapid interaction between the water vapor and the active sites of the CeNi0.89Pd1.1 nanocatalyst [
59]. Moreover, the rate of C–C bond cleavage was improved in the catalytic cracking of heavy oil fractions, and higher amounts of H
2 and O
2 species could be produced from the splitting of steam molecules to react with the asphaltenes and resins over the catalyst surfaces [
59]. At the same time, the increase in asphaltene–CeO
2 interactions enabled the enhanced adsorption and release of oxygen through the Ce
4+/Ce
3+ redox cycle [
60]. Thus, the vacancies of oxygen anions on the surface of the nanocatalyst became unstable [
21,
61], cracking the heavy oil fractions via partial oxidation through the oxygen production promoted by the CeNi0.89Pd1.1 nanocatalyst or water via the redox reaction [
59]. In addition, the redox reaction mechanism between the nanocatalyst, the heavy hydrocarbons, and the H
2O
(g) results in the formation of hydrogen as a byproduct, which could participate in the stabilization of the previously cracked free radicals [
62].
4.6. Crude Oil Characterization before and after Nanofluid Injection
Panels a and b in
Figure 11 show the values of API gravity and asphaltene content, respectively, for the untreated EHO, crude oil after steam injection, crude oil recovered by the dispersed injection of the nanofluid, and after a soaking time of 12 h. As observed, the API gravity increased and the asphaltene content reduced after the steam injection with the CeNi0.89Pd1.1 nanocatalyst. The results showed increases in API of 114.8% and 130% before and after the soaking treatment, confirming that the reaction time between adsorbed heavy fractions and nanocatalysts occurs immediately in the presence of steam. Furthermore, the asphaltene content was reduced by around 72% owing to the catalytic conversion of these compounds by the high catalytic activity of the nanoparticles employed to assist the technology.
Rheological behavior experiments were conducted at 25 °C for oil samples taken after each step of the displacement test.
Figure 12 shows the viscosity of the samples as a function of the shear rate together with the fit of the Cross model. Also,
Table 6 summarizes the estimated rheological parameters found from the regression analysis. In
Figure 12, it is observed for all samples that with the increase in shear rate, the viscosity of the crude oil decreases because of the breakdown of the internal structure of the crude oil, typical of shear-thinning pseudo-plastic fluids. With the injection of the steam without nanoparticles, a decrease in the heavy oil viscosity is appreciable, which could be related to the reduction in the cohesive forces on the molecular structures of combined asphaltene–resin compounds. For the scenarios assisted by nanotechnology, there were more pronounced reductions in viscosity, showing a higher decrease for the crude oil obtained after soaking for 12 h. The viscosity reduction mechanisms associated with the presence of the nanoparticles are the fragmentation and subsequent redistribution of the
n-C
7 aggregate asphaltenes [
63], breaking the bonds and hindering the further formation of the heaviest compounds through the hydrogenation of free radicals and the asphaltene decomposition into lighter components. These results are in agreement with those reported by Franco et al. [
9] and Cardona et al. [
18].
The fitting of the Cross model presents an RMSE% < 1.0. The viscosity at zero (
) and infinity shear rate (
) are consistent with the experimental data, following the order crude oil after steam injection after porous media soaking < crude oil during nanofluid dispersed injection in steam stream < crude oil after steam injection < EHO. Considering the values of
≈ 1, the non-Newtonian behavior of the untreated oil is corroborated. These results are in agreement with those obtained by Franco et al. [
9]. Finally, the degree of viscosity reduction (DVR) was calculated at a shear rate of 10 s
-1 using Equation (1). The results obtained indicate viscosity reductions of 85%, 75%, and 10% for the crude oil recovered after the steam injection post soaking time during the nanocatalyst-based nanofluid injection by the steam stream and steam injection without nanoparticles, respectively. This reduction in viscosity could lead to a reduction in the costs associated with transport and refining operations, mitigation of the environmental impact through the naphtha consumption reduction, as well as an increase in the sale price for crude oil produced.