Insight into the nanoparticle growth in supported Ni catalysts during

bulk Ni surface is facile above 150 °C, hence the growth of. Ni NPs during CO hydrogenation reaction was often as- cribed to the transport of Ni(CO)4...
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Insight into the nanoparticle growth in supported Ni catalysts during the early stage of CO hydrogenation reaction: the important role of adsorbed CO molecules Yunxing Bai, Junfeng Zhang, Guohui Yang, Qingde Zhang, Junxuan Pan, Hongjuan Xie, Xingchen Liu, Yizhuo Han, and Yisheng Tan ACS Catal., Just Accepted Manuscript • Publication Date (Web): 04 Jun 2018 Downloaded from http://pubs.acs.org on June 4, 2018

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ACS Catalysis

Insight into the nanoparticle growth in supported Ni catalysts during the early stage of CO hydrogenation reaction: the important role of adsorbed CO molecules Yunxing Bai,†,‡ Junfeng Zhang,† Guohui Yang,† Qingde Zhang,† Junxuan Pan,† Hongjuan Xie,† Xingchen Liu,*,† Yizhuo Han,*,† Yisheng Tan*,† †

State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, 030001, China ‡

University of Chinese Academy of Sciences, Beijing, 100049, China

KEYWORDS: particle growth, supported metal catalysts, adsorbate coverage, Ni(CO)4 supersaturation, Ostwald ripening

ABSTRACT: In heterogeneous catalysis, reactive gases often accelerate the growth of catalytically-active metal nanoparticles (NPs) by the formation of volatile metal-molecule intermediates, leading to undesired catalyst deactivation. This gasenhanced particle growth is usually descripted by Ostwald ripening, in which the volatile metal-molecule intermediates are assumed to be transported from small to large metal NPs. In this contribution, we demonstrated the strong steric hindrance effect of the adsorbed CO molecules on the transport of Ni(CO)4 molecules between the Ni NPs during the early stage of CO hydrogenation reaction. Extensive analysis of the growth behaviors of different sized Ni NPs revealed a critical concentration for the Ni(CO)4 decomposition on the surface of Ni NPs, which was confirmed by Born Oppenheimer Molecular Dynamics (BOMD) simulations. By considering the existence of the critical concentration, the modified Ostwald ripening model successfully described the main features of particle growth observed experimentally. In addition, the Ni(CO)4 decomposition rate was found to be accelerated by the formation of Ni3C phase because of the weak steric hindrance effect of CO molecules on these surfaces. The role of the adsorbed molecules found herein may provide an important prospective for understanding the growth behaviors of highly dispersed metal NPs in the presence of reactant gases.

INTRODUCTION The growth of catalytically-active metal nanoparticles (NPs) has been found to be one of the dominant causes of deactivation in heterogeneous catalysts.1-3 Due to the excess surface energy, the small metal NPs often behave energetically unstable under reaction conditions, and tend to grow into larger structures.4 The growth of such metal NPs is commonly regarded as a thermally driven process at elevated temperatures.5 However, there is an increasing awareness of the fact that the reactive gases (such as CO and O2) also strongly influence the growth rates through the formation of volatile metal-molecule intermediates in many important industrial catalytic processes.6-12 It was shown that the enhanced growth of Pt NPs in O2 environment could be attributed to the formation of PtO2 molecules.13 Although the transport of metal-molecule intermediates from small to large particles through Ostwald ripening has been proposed, it is still challenging to ascertain the role of these intermedi-

ates in the growth of metal NPs and other mechanism await to be presented. The suppression of Ostwald ripening has been found on Rh NPs by forming stable Rh carbonyl species under high CO pressures and low temperatures.14 Also, recent calculations revealed that the dissociation of PtO2 molecules was inhibited on the O covered Pt surface.15 The growth kinetics has long been associated with the concentration of metal-molecule intermediates which is dependent on the size of metal NPs, however, in the present study, we show that a concentrationdependent deposition process, an issue that has not been raised in previous studies, has an important effect on the resulting growth behaviors of metal NPs.

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Figure 1. TEM images of Ni-3, Ni-6 and Ni-12 before (a, d, g) and after CO hydrogenation reaction (b, e, h). The corresponding evolutions of area-weighted Ni particle size distributions, determined from particle size measurement (at least 5000 particles) in catalysts (c) Ni-3, (f) Ni-6 and (i) Ni-12. The light grey area is guide to the probable pore size distribution of the support.

Ni is one of the most investigated and industrial relevant catalysts, and its stability for CO hydrogenation reaction has received extensive attention for the past decades.16-18 The favorable formation of gaseous Ni(CO)4 molecules has been found to be responsible for the deactivation of Ni catalysts.16 As proved by former researchers, the growth behaviors of Ni NPs were highly dependent on the initial particle size at the early period of low temperature (below 250 °C) reaction in fixed bed reactor.18 The catalyst with well dispersed Ni NPs (4 nm) rapidly lost 90% of its initial activity at first 6 hours, and the huge particles larger than 200 nm were observed over the catalysts. By contrast, the catalyst with larger NPs (8 nm) showed much more stable activity due to limited particle growth during the reaction.19 However, the fundamental understanding of this size-dependent growth behavior of Ni NPs has not been well addressed. Although Ostwald ripening was applied to explain the long-term growth behaviors,17 the formation of the huge Ni particles at the early stage was not well-known.

the catalyst system. Hereby, the deactivation and particle growth of different sized Ni NPs were systematically investigated. We demonstrated the important role of the adsorbed CO molecules played in transport of Ni(CO)4 molecules between the Ni NPs, and we revealed the origin of the formation of huge Ni NPs during the early stage of the reaction.

RESULTS AND DISCUSSION Three Ni/γ-Al2O3 catalysts with different Ni NP sizes (3.2 nm, 6.5 nm and 12.1 nm) were prepared using the incipient wetness impregnation method, and labeled as Ni-3, Ni-6 and Ni-12. The details of sample preparation can be found in SI. The Ni loadings and structure properties of the catalysts are listed in Table S1. TEM micrographs and the corresponding particle size distributions for the reduced catalysts demonstrate that the Ni NPs homogenously dispersed over the catalysts and had relatively narrow Gaussian shapes, with standard deviations of at most 13% of the average size (Figure S1 and Figure 1 a, d, g).

Given that gaseous Ni(CO)4 molecules play an important role in this work, we have employed a continuous stirred tank reactor (CSTR) to evaluate the growth behaviors of Ni NPs at first 10 hours of CO hydrogenation reaction (250 °C, 25% CO/H2, 20 bar) (see Experimental details in Supporting Information (SI)). Wherein, the adoption of high gas flow rate and vigorous stirring in CSTR allows efficient transport of gaseous Ni(CO)4 molecules in

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ACS Catalysis these species increases from Ni-12 to Ni-3. The evolution of CO adsorption spectra with the increasing of CO pressure from 10 mbar to 1 bar, shown in Figure S3, reveals that the primary active sites for the formation of Ni(CO)4 are the unsaturated low coordinated Ni atoms. Thus, owing to the higher density of these sites, small NPs exhibit higher Ni(CO)4 formation rate than the large NPs.

Figure 2. The Ni weight-based activity (not considering the loss of Ni mass) during 10 h on stream at 250 °C with a 25% CO/H2.

Figure 2 shows the weight-based activities of these Ni catalysts for CO hydrogenation at first 10 hours. The initial activities of the three catalysts were proportional to the fraction of high coordinated terrace sites (Figure S2), which is consistent with the results found over Co- and Ru-based catalysts for CO hydrogenation reactions.20,21 After 10 h, the Ni-3 catalyst lost 99% of its initial activity. By contrast, the Ni-6 and Ni-12 catalysts lost 40% and 16% of their initial activities, respectively. This suggests that the deactivation is highly dependent on the initial size of Ni NPs. It is widely accepted that the loss of CO hydrogenation activity of Ni-based catalysts was mainly attributed to the formation of gaseous Ni(CO)4 molecules.16 If the rate of Ni(CO)4 decomposition was slow, these gaseous molecules could be flushed out from the reactor, resulting in Ni loss. However, high Ni(CO)4 decomposition rate leads to dramatically Ni particle growth. Both of these processes caused the loss of active surface area of Ni NPs.

If not considering the Ni(CO)4 decomposition and assuming that the Ni NPs are in equilibrium with Ni(CO)4 molecules, the partial pressure of gaseous Ni(CO)4 depends on the particle radius, R, can be calculated by Gibbs-Thomson equation,18 as follows 2γΩ 4 PNiCO4  K0 exp( )P (1) kB TR CO where K0 is the equilibrium constant corresponding to a bulk nickel crystal, PCO is the partial pressure of CO, γ is the surface tension (2.45 J/m2), Ω is the volume of a nickel atom (10.94 Å3), kB is Boltzmann’s constant, and T is the thermodynamic temperature. Regardless of other factors that influence the catalytic activity, the origin of the sizedependent deactivation phenomenon could be related to the varied Ni(CO)4 concentration (PNi(CO)4) in the catalysts with different sized Ni NPs. To investigate the decomposition of the formed Ni(CO)4 molecules at different concentrations in the three catalysts, we then examined the change of the Ni contents (determined by ICP-AES) in the catalysts at different reaction times. As shown in Figure 4 (a), the Ni contents in three catalysts were found to be decreased during the reaction, revealing slow rate of the Ni(CO)4 decomposition. However, much more Ni loss was found for Ni-12 (20%) than that for Ni-3 and Ni-6 (less than 10%). This is surprising given that higher concentration of Ni(CO)4 molecules were formed on smaller Ni NPs, and more Ni loss should be expected if the similar Ni(CO)4 decomposition rate for different concentration system. We postulate that higher Ni(CO)4 concentration in Ni-3 and Ni-6 exhibit higher Ni(CO)4 decomposition rate than that in Ni-12. It could be tested by the growth behaviors of Ni NPs in the three catalysts.

Figure 3. Operando CO-DRIFT spectra (after subtracting the background due to physically-adsorbed CO gas) at 250 °C in 1 bar CO atmosphere over the reduced catalysts.

To clarifying this size-dependent deactivation phenomenon, diffuse-reflectance IR Fourier-transform spectroscopy was firstly employed to detect Ni(CO)4 formation on the three catalysts at 250 °C in 1 bar CO atmosphere. As shown in Figure 3, a main peak at 2073 cm-1 identified as Ni carbonyl species is observed.22-24 The peak intensity of

Figure 4. (a) The normalized Ni content (closed symbol) and the mean Ni diameter (open symbol) (determined from H2 chemisorption) in the catalysts as a function of reaction time; (b) XRD patterns of fresh and spent catalysts.

Figure 4 (a) shows the changes in the mean Ni diameter (determined from H2 chemisorption) over the catalysts as

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a function of reaction time and Figure 4 (b) shows XRD patterns of the fresh and spent catalysts. The results reveal that the mean Ni diameter in Ni-3 sharply increased from 3.2 nm to 20 nm after 10 h, whereas the Ni-6 displayed a moderately increase from 6.5 nm to 9.6 nm. On the contrary, a size-decrease (from 12.1 nm to 10.8 nm) behavior was observed on the Ni-12 catalyst. These results were further confirmed by TEM characterization shown in Figure 1. Compared with the fresh catalyst (Figure 1 (a) and (d)), remarkable Ni growth was found in Ni-3 and Ni6 after reaction (Figure 1 (b) and (e)). In contrast, considerable quantities of Ni particles smaller than the original particle size were found over the spent Ni-12 (Figure 1 (h) and (i)), demonstrating the loss of Ni in the form of Ni(CO)4 molecules. Moreover, the decrease of the catalytic activity in Ni-12 was well accounted for by the loss of the surface Ni atoms in the catalyst (see Figure S2). This suggests that the deactivation of Ni-12 was attributed primarily to Ni loss, but the particle growth played a major role in the deactivation of Ni-3 and Ni-6. The results further demonstrate the postulation that the Ni(CO)4 decomposition rate is depend on its concentration during the deactivation of the catalysts. From metallurgy, the Ni(CO)4 decomposition on the bulk Ni surface is facile above 150 °C, hence the growth of Ni NPs during CO hydrogenation reaction was often ascribed to the transport of Ni(CO)4 molecules between different sized Ni NPs through Ostwald ripening mode.16,17 However, the inhibiting effect of CO molecules on the Ni(CO)4 decomposition has been observed even at 225 °C in earlier reports.25 This was explained by much more strongly bonding to the Ni surface of CO molecules than Ni(CO)4 molecules. Recently, based on the physical model developed by Munnik et al.,18 the Ni(CO)4 decomposition rate through Ostwald ripening was highly dependent on the CO coverage on the surface of Ni NPs (see SI). Given that that the Ni surface was almost covered by CO molecules (Figure S3) in the present reaction conditions, the fraction of empty sites for the adsorption of Ni(CO)4 molecules was expected to be very small. To achieve an atomistic level understanding of the Ni(CO)4 decomposition mechanism on the CO covered Ni surface, Born Oppenheimer Molecular Dynamics (BOMD) based on self-consistent charge density functional tightbinding (SCC-DFTB) theory was carried out at different Ni(CO)4 concentrations (see SI for detail). Figure 5 shows the situation when the Ni(CO)4 concentration is low (corresponding with the situation in Ni-12). In this case, the surface is initially covered by many CO molecules, and the Ni(CO)4 has a low chance of approaching the surface; it is initially positioned physisorbed to the surface through the CO molecules. When the system is heated (0.5 ps), one could see that some CO molecules started to desorb from the surface. At 0.8 ps, the Ni(CO)4 lost one of its CO ligand due to the collision with the surrounding CO molecules, and became Ni(CO)3. However, because of the strong steric hindrance effect from the remaining CO molecules on the surface, the Ni(CO)3 remains floating above the surface in its current form for the duration of

the BOMD simulation, and no further decomposition was observed. This result reveals that the Ni loss in Ni-12 was related to the suppression of the decomposition of low concentrated Ni(CO)4 molecules on the Ni surface due to the steric hindrance effect from the adsorbed CO molecules.

Figure 5. Representative BOMD configurations showing the initial (a), intermediates (b, c, d), and final (e) states of the Ni(CO)4 decomposition on the Ni (100) surface at low concentration leading to non-deposited Ni(CO)3. The Ni, C, and O atoms are in grey, brown, and red color, respectively.

In order to gain insight into the underlying decomposition mechanism of high concentrated Ni(CO)4 molecules in Ni-3 and Ni-6, the detailed size evolutions were analyzed from TEM micrographs shown in Figure 1 (a)-(f). Careful examination of different areas with different image magnifications for the spent Ni-3 and Ni-6 catalysts reveals that a bimodal system consisting of small particles (SPs) and large particles (LPs) was developed (Figure S4 and S5). As shown in Figure 1 (b) and (e), LPs (labeled with red circles) were found to be extremely larger than their neighboring SPs (labeled with white circles), which was further supported by the changes of size distributions in Ni-3 and Ni-6 (Figure 1 (c) and (f)). To make clear understanding of the features of SPs and LPs in Ni-3 and Ni-6, their mean surface diameters and particle densities (number of particles per surface area) were determined from the bimodal size distributions and TEM micrographs, respectively. The results are summarized in Figure 6 and Table S2. As shown in Figure 6 (a), the mean diameters of the SPs were comparable with the values of Ni NPs in the fresh catalysts and only exhibited slightly change during the reaction. However, their particle densities, shown in Figure 6 (b), gradually decreased during the reaction, and about 99% and 43% SPs in Ni-3 and Ni-6 diminished after 10 h, respectively. The particle density decrease rates of SPs in the catalysts are consistent with the formation rate of Ni(CO)4 molecules over different sized Ni NPs (Figure S6). This result further

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ACS Catalysis demonstrates the suppression of Ni(CO)4 decomposition for SPs in Ni-3 and Ni-6. It also suggests that the growth process in Ni-3 and Ni-6 was attributed to the formation of LPs which was related to the decomposition of Ni(CO)4 molecules evaporated from the diminished SPs. Based on the evolution of the density of LPs, shown in Figure 6 (b), the particle growth process was divided into two stages during the 10 h reaction. At first 1 h, the LPs sharply formed in Ni-3 and Ni-6, and then grow up (Figure 6 (a)) with their particle densities kept constant afterward. It suggests that the formation of LPs is related to the fast decomposition of high concentrated Ni(CO)4 molecules at the early stage of reaction, and then they continued grow up by consuming the remaining SPs at the later stage of the growth process.

Figure 7. Representative BOMD configurations showing the initial (a), intermediate (b, c, d), and final (e) states of the Ni(CO)4 decomposition on the Ni (100) surface at high concentration leading to surface-deposited Ni(CO). The Ni, C, and O atoms are in grey, brown, and red color, respectively.

Figure 6. The features of small particles (SPs) and large particles (LPs) during the reaction. (a) The mean diameter of SPs and LPs calculated from the bimodal shape particle size distributions of the catalysts; and (b) their particle density (number of particles per surface area) determined from TEM micrographs; see SI.

To understand the mechanism of the formation of LPs at the early stage, Figure 7 shows the results of BOMD simulations for the decomposition of high concentrated Ni(CO)4 molecules on the CO covered Ni surface (corresponding with the situation for Ni-3 catalyst, see Figure S11). High concentrated Ni(CO)4 molecules have a higher chance of approaching the surface, and they were initially positioned at a chemisorption distance to the surface. At 0.4 ps, we can already observe the decomposition of one of the Ni(CO)4 molecules to Ni(CO)2 on the surface. At 2.1 ps, the Ni(CO)2 lost another CO ligand, and became Ni(CO); meanwhile, the second Ni(CO)4 decomposed to Ni(CO) and sit next to the first Ni(CO) fragment. Then, the third Ni(CO)4 molecule decomposed to Ni(CO) on the surface at 2.8 ps. The three decomposed Ni atoms are close to one another and form a triangle on the Ni surface. It is clear that the decomposition of high concentrated Ni(CO)4 molecules involves competitive adsorption of Ni(CO)4 and CO on the Ni surface, followed by the successive chemisorption of the carbonyl groups on neighboring empty surface Ni sites. The result also suggests that the deposition of Ni atoms is an instant and almost barrier-less process when enough empty sites were satisfied for the decomposition of Ni(CO)4.

What we have observed in the BOMD simulations in Figure 5 and Figure 7 are two extremes: the floating of the Ni(CO)3 fragments above the surface without Ni deposition to the surface at low Ni(CO)4 concentration, and the fast deposition of Ni on the surface at high Ni(CO)4 concentration. At low concentration, the Ni(CO)4 decomposition was inhibited by the steric hindrance effect of adsorbed CO molecules on the Ni surface. However, with the increase of the Ni(CO)4 concentration, the probability of Ni(CO)4 molecules approaching to the Ni surface increases. Then, the steric hindrance effect of CO molecules for the Ni(CO)4 adsorption decreases at high Ni(CO)4 concentration because of the decrease of CO concentration close to the Ni surface. Given the certain fraction of empty surface sites required for the chemisorption and decomposition of Ni(CO)4 molecules, it is therefore reasonable to conclude that there must exist a critical Ni(CO)4 concentration, above which the deposition of Ni atoms on Ni surface can happen. To further understand the concentration-dependent decomposition process, the Ni(CO)4 concentration, independently of the CO pressure, could be transformed to the Ni(CO)4 supersaturation ratio, S, defined as PNi(CO)4/(K0PCO4), where PNi(CO)4 is the Ni(CO)4 concentration in the catalyst system at certain reaction time. For an individual Ni NP with radius R, if we assume that it is in equilibrium with the gaseous Ni(CO)4 molecules, the equilibrium Ni(CO)4 supersaturation ratio (SR) can be calculated as exp( 2γΩ⁄kB TR ) based on equation (1). If not considering Ni(CO)4 decomposition, the mean-field equilibrium Ni(CO)4 supersaturation for all particles in the catalysts was labeled as Smax, and it is equal to the areaweighted average of SR of all particles in the catalysts (see SI for details). The calculated value in the three catalysts are Smax = 15 for Ni-3, Smax = 3.6 for Ni-6 and Smax = 1.4 for

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Ni-12. The presence of LPs in Ni-3 and Ni-6 indicates that the driving force is sufficient in these two catalysts, whereas the generated Ni(CO)4 molecules in Ni-12 are flushed out from the reactor due to the low S value. The critical S value for Ni(CO)4 decomposition in the present work was justified experimentally as follows. Note that the features of LPs in Ni-3 and Ni-6 exhibited very different. As shown in Figure 6 (b), the density of LPs in Ni-3 was higher than that in Ni-6. However, the mean diameter of the LPs in Ni-6 was larger than that in Ni-3 (Figure 6 (a)). After 10 h, the mean diameter of LPs reached 20 nm in Ni-3, but the mean value in Ni-6 was up to 47 nm. As the average diameter of the support is 19 nm, the location of these LPs in the two catalysts is expected to be different. The TEM results in Figure 4 (c) and (f) reveal that the majority of the LPs over Ni-3 were confined in the pores of the support; nevertheless, the LPs in Ni-6 grow larger than the mean pore diameter of the support. SEM images of macroscopic catalyst grains before and after reaction show that large amount of LPs were found to be located on the external surfaces of catalyst Ni-6 grains (see Figure S7 for detail). The different locations of LPs over the Ni-3 and Ni-6 catalyst grains revealed the different deposition sites where the critical level was satisfied. High S value in Ni-3 results in the formation of the LPs throughout the catalyst grains; however, the deposition process in Ni-6 mainly occurs on the external surface of the catalyst grains. Note that mechanical stirring (1200 rpm) and gas flow rate (800 sccm) in CSTR can have a significant impact on the Ni(CO)4 decomposition process, since both actions could produce fluctuations of Ni(CO)4 concentration on the surface of the catalyst grains. Thus, we hypothesize that the enrichment of LPs on the external surface of Ni-6 grains was attributed to the local increase in Ni(CO)4 concentration where the S reached the critical value. To verify this hypothesis, we studied the growth behaviors of Ni-3 and Ni-6 in fixed bed at very low CO flow rate (5 sccm) where only slightly fluctuations of Ni(CO)4 concentration occurred. As expected, significant decrease in the density of LPs was observed at the surface of Ni-6 grains and 50% Ni lost after 6 h, while large amount of huge particles were found on Ni-3 grains (see Figure S9). Given the mean-field equilibrium Smax was 3.6 in Ni-6, the formation of LPs on the external catalyst grain surface suggested that the critical level of S for Ni(CO)4 decomposition should be above 3.6. Considering the existence of the critical S value, based on the physical model developed by Munnik et al.18, the Ni(CO)4 decomposition rate on the surface of Ni NPs with radius R obeys the following form 0, S <  2γΩ r  2 (2) KθE S-exp( ) , S ≥  k B TR

where θE is the fraction of empty adsorption sites; S is the Ni(CO)4 supersaturation ratio defined as PNi(CO)4/(K0PCO4); Sc is the critical supersaturation value for Ni(CO)4 decomposition on Ni surface; and K = kK0PCO4, where k is

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the rate constant (see SI). Under certain reaction conditions, if the value of S in the catalyst system below Sc, the Ni(CO)4 decomposition rate is zero. If the value of S in the catalyst system equal or above Sc, the value of exp( 2γΩ⁄kB TR ) indicates the equilibrium supersaturation ratio of an individual Ni NP with radius R, and it can be labeled as SR. The Ni(CO)4 decomposition rate on the surface of Ni NPs was depend on the fraction of the empty sites for the adsorption of Ni(CO)4 molecules and the difference between SR and S. For the Ni NPs of radius R with the value of SR larger than the S value in the system, r is negative, and the Ni(CO)4 molecules leave the NP to the gas phase. For the Ni NPs of the radius R with the value of SR less than the S value in the system, r is positive; the Ni(CO)4 molecules tend to captured by the NP, and the Ni NP size increases. Thus, the difference between SR and S determines the growth direction of the individual NP of interest. At the early stage, the concentration of Ni(CO)4 molecules emitted from Ni NPs in the catalysts accumulated with the reaction time, and the S value in the catalyst increased. Three situations were considered based on the relative value between the equilibrium state (Smax) and the critical state (Sc). In the case of Smax < Sc, such as Ni-12 system, the generated Ni(CO)4 molecules were flushed out from the reactor, resulting in the decrease of mean diameter of Ni NPs. In the case of Smax ≥ Sc, when the S in the catalyst system reached to the critical value Sc, the Ni NPs with SR above Sc tend to emit the Ni(CO)4 molecules and shrink in size, while the Ni NPs with SR below Sc are able to capture the Ni(CO)4 molecules and grow up. This is the situation in Ni-3 catalyst system. Based on the non-uniform of the initial particle size distribution in the catalysts, the SR varies between 3.3 and 180 in Ni-3. As the S reached to Sc, only those Ni NPs with SR between 3.3 and Sc could grow up. Given that the critical value Sc estimated from the TEM/SEM results and the relative frequency of NPs in the different size ranges over Ni-3, the fraction of the Ni NPs that could grow up was expected to be very small (below 5%, see Table S3). This is consistent with the particle density of LPs (~2.2% in all particles) formed in Ni-3. These LPs will grow up by consuming the Ni(CO)4 molecules emitted from the SPs (SR > Sc). The results explain why the density of the formed LPs kept constant and the density of SPs gradually decreased during the reaction. With the size of these LPs increases, they tend to grow up more rapidly. This results in the development of a bimodal size distribution consisting of SPs and LPs. In the situation of Ni-6, the Smax was slightly below the Sc. Only those Ni NPs on the external surface of catalyst grains where the S reached the Sc could grow up because of the strong local fluctuation in Ni(CO)4 concentration. These few NPs (~0.1%) gradually grow up by consuming abundant Ni(CO)4 molecules to very large size (up to 200 nm). It is worth noting that the Ni(CO)4 concentration (PNi(CO)4) was estimated from Gibbs-Thomson relation in

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still happens (Figure S13). The results reveal that the steric hindrance effect of CO molecules on the carburized surfaces is markedly weaker than that on the Ni surface, and the adsorption and decomposition of Ni(CO)4 molecules was facilitated on the carburized surfaces. Thus, with the decrease of the Ni(CO)4 concentration below the critical value for Ni NPs at the later stage of the reaction, the carburized LPs continued to grow up by capturing the Ni(CO)4 molecules formed from the remaining SPs. This further explains the bimodal size distribution developed during the growth process. Moreover, temperature-programmed hydrogenation (TPH) was used to hydrogenate the carbon species in the spent catalysts into methane (see Figure S8). The results reveal that a peak at 338 °C, assigned to the hydrogenation of Ni3C phase, was observed in Ni-3 and Ni-6; however, no peak at 338 °C was found in Ni-12. This result explains why there was no growth of Ni NPs in Ni-12 during the reaction. Previous work has revealed that the CO dissociation was accelerated by the decomposition of Ni(CO)4 on the Ni surface, resulting in the formation of Ni3C phase.26,27 However, detailed mechanism about the relation between the decomposition of Ni(CO)4 and the formation of Ni3C phase needs further experimental and theoretical work to identify the intermediate surface phases.

CONCLUSIONS

Figure 8. (a) HRTEM and (b) HAADF-STEM images of one large Ni particle, and (c)-(f) corresponding EDS mapping images of (b) for C, O, Al, and Ni elements, respectively.

As a final aside, it is worth noting that the growth of LPs consumed abundant of Ni(CO)4 molecules at the early stage, resulting in the Smax value in the catalyst system gradually decreased. When the Smax value was below Sc, the suppression of the Ni particle growth was expected. However, at the later stage of the growth process, the LPs continued grow up (Figure 6), and no significant Ni loss was observed in Ni-3 and Ni-6 (Figure 3 (a)). This was explained by the surface carburization of LPs (see Figure S8 and Figure S2). The representative HRTEM and EDS mapping of the LPs, shown in Figure 8, reveal that the surface of LPs was covered by the Ni3C phase. This was also confirmed by the appearance of XRD patterns of Ni3C phase in the catalysts after reaction at 35 bar (see Figure S10). To address the role of the Ni3C phase at the later stage of the growth process, similar BOMD simulations were performed on C-terminated Ni3C (111) surface (Figure S12 and S13). As expected, the simulation with high Ni(CO)4 concentration shows the fast deposition of Ni atoms on the surface (Figure S12). Surprisingly, even with low Ni(CO)4 concentration, the decomposition of Ni(CO)4

The growth behaviors of different sized Ni NPs over γAl2O3 support were studied at the first 10 hours of CO hydrogenation reaction (250 °C, 25% CO/H2, 20 bar) in CSTR. A combination of analyzing catalyst deactivation, Ni contents, CO-DRIFT spectra, and TEM results provided a consistent picture of a concentration-dependent Ni(CO)4 decomposition process during the growth of Ni NPs at the early stage of the reaction. The BOMD simulations demonstrated the strong steric hindrance effect of the adsorbed CO molecules on the adsorption and decomposition of Ni(CO)4 molecules on the Ni surface. The results suggested a critical concentration for the Ni(CO)4 decomposition on the surface of Ni NPs, which was demonstrated by the TEM/SEM results. By considering the existence of the critical concentration, the modified Ostwald ripening model successfully described the main features of particle growth observed experimentally. Moreover, due to weaker interaction with CO molecules than that of Ni surface, the formation of Ni3C phase facilitated the Ni(CO)4 decomposition process. In general, our findings provide a new insight for understanding the reactive gas-induced growth of metal NPs in the deactivation of supported metal catalysts.

ASSOCIATED CONTENT Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.” Experimental and computational details, structure of the catalysts, analysis of catalytic activity, in-situ CO-DRIFT spectroscopy, additional TEM images, SEM images, TPH

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results of the spent catalysts, additional results of BOMD simulation, and Ostwald ripening model.

AUTHOR INFORMATION Corresponding Author * [email protected]; * [email protected]; * [email protected]

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was supported by Shell Global Solutions International BV. The authors are also grateful to Dr. Yequn Liu for TEM and ultramicrotomy instructions. We thank Dr. Xiaofeng Gao for kind help in creating the mechanism scheme. We are grateful to Dr. Peng Wang for drawing the table of content. We are also thankful for valuable discussions with Prof. Zhangfeng Qin, Prof. Xiangyun Guo, Dr. Xiaoning Guo, Dr. Minghui Tan, Dr. Tao Zhang, Dr. Kai Sun and especially Dr. Pengju Yang.

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