Insights into the CO Formation Mechanism during Steam Reforming of

Article Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

pubs.acs.org/IECR

Insights into the CO Formation Mechanism during Steam Reforming of Dimethyl Ether over NiO/Cu-Based Catalyst Feiyue Fan,† Long Zhao,*,† Hong Hou,*,† and Qi Zhang‡ †

State Key Laboratory of Environmental Criteria and Risk Assessment, Chinese Research Academy of Environmental Sciences, Beijing 100012, PR China ‡ Department of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, PR China

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S Supporting Information *

ABSTRACT: Steam reforming of dimethyl ether on Cubased catalyst was experimentally studied. It is found that formation of H2 and CO2 occurred over Cu/γ-Al2O3/Al, while methanol decomposition to CO and H2 was the main reaction over Cu/Ni/γ-Al2O3/Al in methanol steam reforming (MSR), which is similar to that on metallic Ni. However, XPS results show that nickel species existed as an oxidation state (NiO) over Cu/Ni/γ-Al2O3/Al. Therefore, reaction pathways of MSR over Cu(111) and NiO/Cu(111) were further explored using theoretical density functional theory calculations, and it was found that different reactivities of CH2O* resulted in the distinct product selectivity. On Cu(111), CH2O* reacting with OH* to CH2OOH* is favored, while CH2O* prefers to dehydrogenate to H* and CO* directly on NiO/Cu(111). This provided further evidence on the experimental results and fundamental understanding on the role of NiO species in affecting the MSR mechanism, which was helpful for rational design of selective Cu-based catalysts. Different solid acid catalysts, especially γ-Al2O3, zeolites, and WO3/ZrO2,8−10 were applied to catalyze the hydrolysis of DME (eq 2), which was reported as the rate-controlling step for the DME SR process.11 Compared with other catalysts, γ-Al2O3 shows better DME hydrolysis performance without undesirable byproducts due to its lower acidity and is the more appropriate one. However, high temperatures (>300 °C) are generally needed for DME hydrolysis over γ-Al2O3.12 Cu-based catalyst is commonly used in MSR (eq 3) because of its low cost and high activity, whereas it has low thermal stability and poor catalytic performance, since copper species are gradually aggregated at temperatures above 300 °C.13−17 Then, the component Ni was found to be effective for suppressing copper sintering and enhanced stability of Cu-based catalysts dramatically.18 Nevertheless, the nickel-modified Cu-based catalysts showed high byproduct yields with ca. 26% CO content in DME SR, and the CO formation mechanism was not reported. Up to now, despite the numerous investigations on the MSR reaction, the reaction mechanism was still in controversy, since the intermediates could not be completely detected by experiment.19,20 Recently, the reaction mechanisms addressed using density functional theory (DFT) calculations have attracted more and more attention due to the helpfulness in

1. INTRODUCTION The increasing exhausting of traditional fossil fuels and asinduced seriously environmental problems has boosted the development of new alternative energies. Hydrogen is regarded as one of the most promising clean energies because of its easy availability and nonpollution; however, the storage and transportation of hydrogen are still a big challenge.1 Thus, onsite hydrogen production via catalytic reforming of hydrogenrich compounds (hydrocarbons, alcohols, and biomass) is proposed.2−5 Compared with other compounds, dimethyl ether (DME) is an attractive one which possesses many advantages such as nontoxicity, high hydrogen content, similar physical properties with LNG, and ease of distribution.6,7 Steam reforming of DME (eq 1, DME SR) usually comprises two consecutive steps, namely, the hydrolysis of DME (DME HYD) to methanol on acidic catalysts (eq 2) and subsequent methanol steam reforming (MSR) to CO2 and H2 on metallic catalysts (eq 3). Thus, bifunctional catalysts containing acidic function and metallic function are necessary for DME steam reforming. DMESR:

CH3OCH3 + 3H 2O ↔ 2CO2 + 6H 2

DMEHYD: CH3OCH3 + H 2O ↔ 2CH3OH MSR:

CH3OH + H 2O ↔ CO2 + 3H 2

ΔHr0 = + 135 kJ/mol

(1)

ΔHr0 = + 37 kJ/mol

(2)

ΔHr0 = + 49 kJ/mol

(3)

© XXXX American Chemical Society

Received: June 12, 2018 Revised: December 26, 2018 Accepted: February 1, 2019

A

DOI: 10.1021/acs.iecr.8b02628 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

X-ray photoelectron spectroscopy (XPS) was carried out to measure the chemical state of catalysts by an ESCALAB 250 analyzer (Thermo Scientific, Al Kα radiation source). Calibration of binding energy was performed by C 1s peak (284.6 eV). 2.3. Catalytic Activity Measurement. The performances of copper-based and nickel-modified samples in DME SR were evaluated in a tube reactor with an inner diameter of 12 mm at normal pressure. About 6 mm2 catalyst pieces mixing with 20− 40 mesh Raschig rings were loaded into the reactor. The temperatures of the catalyst bed were measured by a thermocouple placed in the reactor. Prereduction treatments of catalysts by hydrogen were not performed before reaction, unless otherwise noted. H2O, vapored at 150 °C by a preheater, along with dimethyl ether was supplied to the catalyst bed. An online GC equipped with detectors of TCD (thermal conductivity detector) and FID (flame ionization detector) was used to analyze the components in reactants and products. Before analysis, a condenser was used to trap water in the influent and effluent gases. The compositions of N2, CO2, and CO were separated by a TDX-01 column, while the compositions of CH4, CH3OH, and DME were separated by a PORAPAK-Q column. Conversion of DME (eq 4) and product selectivity (eq 5) are calculated on the basis of the following definitions

understanding heterogeneous catalysis on the molecule level. Indeed, many works19−28 have been reported on the MSR mechanism over several surfaces (Cu(111), Pd(111), PdZn(111), Co(111), NiZn(111), etc.) on the basis of DFT calculations and summarized in Table S1 (Supporting Information). It can be seen that the CH2O* intermediate preferentially reacts with OH* to produce CH2OOH* on Cu(111), resulting in H2 and CO2 as main products. Meanwhile, similar pathways of MSR on Cu(111) and PdZn(111) could be observed, which agrees with experiment results.13 However, on Pd(111) and Co(111) (group 8−10) surfaces, direct dehydrogenation of CH2O* is favored and leads to CO and H2 as the main products. This is in agreement with experimental result.29 However, the mechanistic study on the mechanism of MSR over CuNi surfaces was rarely reported. Moreover, the valence states of nickel species and their contribution to the pathways of the MSR step in the DME SR reaction system were not clearly understood yet. In this work, Cu-based catalyst supported on a plate-type γAl2O3/Al monolith was developed. The effect of nickel on the product distribution of copper-based catalyst during DME SR was experimentally investigated. To understand the source of different selectivities in DME SR over copper-based and Nidoped catalysts, the reaction pathways of MSR (the second step in the DME SR process) over Cu(111) and NiO/Cu(111) were systematically identified using theoretical DFT calculations. The optimizing configurations as well as the corresponding adsorption energies for pertinent species over these two surfaces were first discussed. Then, the elementary reactions involving activation and reaction energies during MSR were extensively studied. Finally, the reaction mechanisms of DME SR on Cubased and nickel-modified samples were proposed.

ij yz n Conversion of DME (%) = jjjj1 − DME zzzz × 100% nDME,0 { k Product selectivity (%) =

ni × 100% ∑ ni

(4)

(5)

in which nDME,0 and nDME represent the inlet and outlet molar flow rates of dimethyl ether; ni refers to the molar flow rate of the products (H2, CO2, CO, and CH4). 2.4. Computational Methods. DFT calculations in the study were conducted by the Vienna ab initio Simulation Package (VASP).30 The projected augmented wave (PAW) method is applied to describe the interactions of electrons and the ionic core with a 400 eV energy cutoff.31 The exchangecorrelation GGA-PW91 functional is performed to calculate electronic structure.32 To verify the accuracy of this functional, the adsorption energies of several MSR intermediates in this work were compared with previous experimental and theoretical values on the Cu(111) surface. As shown in Table S2 (Supporting Information), the adsorption energies based on the theoretical PBE and PW91 functionals were slightly weaker than the experimental results.33−35 However, for theoretical values, the PW91 results obtained in this work were similar to previous PBE and PW91 values in the literature,19,25,28,36 which demonstrated the accuracy of the PW91 functional. The calculations for the Cu(111) surface were performed on a 3 × 3 unit cell. A mesh of 4 × 4 × 1 k-point is applied to sample Brillouin zone with 0.1 eV Fermi level smearing.37 The lattice constant of bulk copper is optimized to 3.63 Å, which agrees well with the experimental value of 3.62 Å. A four-layer slab with the bottom two layers fixed to the bulk position during calculations was modeled for the Cu(111) surface. A 14 Å vacuum spacing was introduced along the z direction. The NiO/Cu(111) was modeled with a nanoparticles chain deposited over the Cu(111) surface (4 × 4 unit cell with four layers), which is similar for the oxide/Cu system.38,39 A 14 Å vacuum spacing was introduced along the z direction. Atoms in

2. EXPERIMENTAL SECTION 2.1. Preparation of Catalysts. A structured anodic alumina (γ-Al2O3/Al) support was developed through the anodization method using an Al plate (1060, 45 mm × 150 mm × 0.445 mm) as the substrate. The solutions of NaOH (4 min, 10 wt %) and HNO3 (2 min, 10 wt %) were first used for the pretreatment of the aluminum plate. Subsequently, this substrate was treated at 50 A/m2 (current density) for 12 h at a temperature of 20 °C in an oxalate solution (0.4 mol/L), and then, a porous Al2O3 layer was formed on the surface of the substrate. A small amount of oxalate that remained in this substrate was removed by calcination at 350 °C for 1 h in air. Afterward, deionized water treatment for 1 h was performed at 80° C. Calcination at 500 °C was finally conducted in air for 4 h, and monolithic γ-Al2O3/Al was obtained. The impregnation method was used for the preparation of Cu/γ-Al2O3/Al. The anodic alumina monolith was immersed for 12 h in 1.5 M copper nitrate solution at 25 °C, followed by natural drying. Then, calcination for 4 h was carried out at 500 °C in air. The sequential impregnation method was performed to prepare Cu/Ni/γ-Al2O3/Al. The resultant Cu-based catalyst was immersed for 12 h in 1.0 M nickel nitrate solution at 25 °C, followed by natural drying. Then, calcination for 4 h was carried out at 500 °C in air. 2.2. Characterization of Catalysts. Inductively coupled plasma spectroscopy (ICP, Varian 725-ES) was conducted to analyze the loadings of Cu and Ni, which were calculated according to the Al2O3 layer amount. B


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Industrial & Engineering Chemistry Research the bottom two layers were fixed in the bulk position during calculations. A mesh of 5 × 5 × 1 k-point is applied to sample Brillouin zone with 0.1 eV Fermi level smearing. Spin-polarized calculations were performed for the NiO/Cu(111) catalyst. The adsorption energy (Eads) is calculated by the following definition Eads = Etotal − Eadsorbate − Esubstrate

CH3OCH3 → H 2 + CO + CH4

(9)

CO2 + 4H 2 ↔ CH4 + 2H 2O

(10)

CO + 3H 2 ↔ CH4 + H 2O

(11)

It was also found from Figure 1b that the H2/CO molar ratio was about 2 for CatCuNi catalyst, which evidenced that methanol decomposition (eq 7) was the main step. This is similar to that on group 8−10 metals (metallic Ni, Pt, Pd, etc.), where it was reported that CO and H2 are predominantly produced via methanol decomposition.29,43 In addition, the CH4 selectivity was still trace even though the component nickel existed. To further identify the valence state of metal components in the prereduced CatCuNi sample, XPS analysis was performed. The Cu 2p spectra of CatCuNi (Figure 2a) showed a main peak at

(6)

in which Etotal, Eadsorbate, and Esubstrate are the energy of the substrate with the adsorbate, the adsorbate in the gas phase, and the substrate, respectively. Transition states (TSs) were determined by the climbingimage nudged elastic band (CI-NEB) method,40,41 and the convergence criteria of force and energy were 0.03 eV/Å and 10−4 eV, respectively. Zero-point energy (ZPE) corrections were computed by vibrational frequencies.

3. RESULTS AND DISCUSSION 3.1. Catalytic Performance of Cu-Based Catalyst in DME SR. Figure 1 shows the results of the thermal stability of

Figure 2. XPS spectra of CatCuNi catalyst: (a) Cu 2p; (b) Ni 2p.

ca. 932.5 eV, suggesting the existence of Cu0/Cu+ species.44 However, a minor satellite peak at around 943.0 eV belonging to Cu2+ was also observed, which is mainly resulting from reoxidation of reduced Cu by air before measurement. Figure 2b shows the Ni 2p spectra of CatCuNi catalyst. It was found that CatCuNi showed only a satellite (about 862.1 eV) and a main peak (about 856.0 eV), implying the existence of NiO species,45 which was due to the irreducibility of nickel oxide below 420 °C.18 This also explained the fact that CH4 was trace over CatCuNi, since NiO species could effectively suppress the methanation reactions of COx (eq 10 and eq 11).38,46 As discussed above, for the MSR step, the product distribution of CatCuNi was similar to that on metallic group 8−10 (Ni, Pd, etc.). However, all of the nickel species existed as an oxidation state (NiO) on the CatCuNi surface during reaction. To the best of our knowledge, the relative research was rarely reported and the phenomenon observed in this study could not be explained by the available literature. Therefore, theoretical DFT calculations were further applied to investigate how the addition of nickel oxide species affected the reaction pathways of MSR over Cu-based catalyst. For comparison, the reaction mechanism of MSR over Cu(111) was also studied. 3.2. Reaction Mechanisms of MSR on Cu(111) and NiO/ Cu(111). 3.2.1. Adsorption of Intermediates. Table S3 (Supporting Information) listed the optimizing geometries and adsorption energies for various species on Cu(111) and NiO/Cu(111) during MSR. The adsorption configurations were displayed in Figures 3 and 4. It was found that H2O*, OH*, and O* preferentially adsorb on Ni species, while H* tends to adsorb on O species over the NiO/Cu(111) surface. Except for H*, O*, OH*, and H2O* species, all of the other intermediates over Cu(111) as well as NiO/Cu(111) surfaces display similar geometries and configurations.

Figure 1. (a) Time-on-stream profiles of DME conversion; (b) H2 and C1 (CO2, CO, and CH4) selectivity at the first hour. Reaction conditions: n(DME):n(H2O) = 1:4, total flow rate = 3600 mL/(g·h), T = 350 °C.

Cu-based catalyst in DME SR, which was studied in our previous work.18 The components Cu and Ni supported on anodic alumina were expressed as “CatCuNi” for convenience. It can be seen from Figure 1a that CatCu shows a low durability at 350 °C because of metallic copper sintering. Meanwhile, the conversion of DME is only about 37% over CatNi, which is similar to the value (36%) of DME hydrolysis conversion on anodic alumina support.42 This indicates CatNi could not catalyze the MSR (eq 3) process. However, the presence of nickel improved the stability of Cu-based catalyst considerably, and deactivation was not observed over CatCuNi catalyst during reaction. In the present study, reaction mechanisms of DME SR on CatCu and CatCuNi catalysts were further investigated by experimental and theoretical methods. As shown in Figure 1b, for the CatCu catalyst, H2 and CO2 were produced together as the main products in a molar ratio of ca. 3 (H2/CO2), which indicated that the step of MSR proceeds not via CH3OH decomposition (eq 7) but through formation of H2 and CO2 (eq 3). Moreover, the selectivity to CH4 was trace, suggesting a minor contribution of CO2 formation (eq 8), DME decomposition (eq 9), and CO2 and CO methanation (eqs 10 and 11). CH3OH → CO + 2H 2

(7)

CH3OCH3 + H 2O ↔ CH4 + 2H 2 + CO2

(8) C


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Figure 3. Side and top views of the most stable adsorption geometries of MSR intermediates on the Cu(111) surface. Atom color labeling: orange, Cu; gray, C; red, O; white, H.

It also can be seen from Table S3 (Supporting Information) that the saturated intermediates (CH3OH*, CH2O*, H2O*, CHOOH*, and CO2*) are bound weakly to the surfaces with small adsorption energies below −0.27 eV, which was attributed to their closed-shell electronic structure.47 However, the unsaturated species (CH3O*, CH2OH*, CH3*, CHO*, CH2OOH*, OH*, etc.) adsorb strongly on the surfaces with larger adsorption energies (> −1.0 eV). On the other hand, the saturated CO also interacts strongly with Cu(111) and NiO/ Cu(111), with ca. −0.88 eV adsorption energy. The donation− back donation interactions of substrate and CO resulted in strong CO adsorption.48 3.2.2. Reaction Pathways. The reaction energies and barriers for elementary steps on Cu(111) and NiO/Cu(111) during MSR are summarized in Table S4 (Supporting Information). The corresponding energetics of MSR is shown in Figure 5. Moreover, the TS structures with geometric parameters are displayed in Figures 6 and 7. 3.2.2.1. H2O Dissociation. On the Cu(111) surface, H2O* decomposition into H* and OH* (R1) is thermoneutral (Er = −0.01 eV) with a 1.16 eV barrier, which agrees with 1.15 eV reported in the literature.49 In this step, OH* and H* dissociated from H2O* (top-site) moved to nearby fcc sites via TS1 (Figure 6), in which the O−H bond length is stretched from 0.99 to 1.65 Å. For OH* dissociation (R2), the length of O−H gradually increases from 0.99 to 1.58 Å at the transition

state (TS2, Figure 6). Then, the dissociating O* and H* adsorb on nearby fcc hollow sites. The corresponding reaction and activation energies are 0.58 and 1.68 eV, respectively, which indicates that the process (R2) could be very difficult to take place because of the quite high endothermicity and barrier. On the NiO/Cu(111) surface, H2O* dissociation (R1) is endothermic (Er = 0.45 eV) with a much more facile 0.57 eV barrier, indicating the presence of Ni species facilitates the dissociation of H2O*. In addition, the value of 0.57 eV agrees with the previously reported 0.59 eV.38 At the transition state (TS1, Figure 7), the breaking O−H distance is extended from 0.99 to 1.58 Å. After reaction, H* adsorbs on an adjacent O site, while OH* still adsorbs on the Ni site. For OH* dissociation (R2), the O−H length gradually increases to 1.55 Å from 0.99 Å in the TS (TS2, Figure 7). The produced H* moves to a nearby O site, while O* still adsorbs on the Ni site after dissociation. The step is calculated to be strongly endothermic (Er = 0.71 eV) with an activation energy of 1.53 eV. Thus, this process is probably unfeasible on NiO/Cu(111), which is similar to Cu(111). In addition, considering the coverage of OH* species and reaction balance, OH* disproportionation (OH* + OH* → O* + H2O*) will be unfavorable. Thus, the O* species on Cu(111) and NiO/Cu(111) was not considered in this work. 3.2.2.2. CH3OH Decomposition to CH2O. On the Cu(111) surface, methanol dehydrogenation has two possible pathways: D


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Figure 4. Side and top views of the most stable adsorption geometries of MSR intermediates on the NiO/Cu(111) surface. Atom color labeling: orange, Cu; blue, Ni; gray, C; red, O; white, H.

Comparing with R5 and R6, it is found that the CH3O* preferred to be processed via R5 rather than the formation of CH3* through R6 because of the lower activation barrier, which is consistent with previous DFT results.25 In addition, since CH4 selectivity in the products is determined by CH3* hydrogenation,50 the higher barrier for CH3* formation (R6) is essential to avoid the formation of CH4. This further explained the origin of the trace CH4 formation over Cu-based catalyst, as shown in Figure 1. On the NiO/Cu(111) surface, similar to Cu(111), two possible pathways for CH3OH* dehydrogenation were also compared. For the O−H bond cleavage of methanol (R3), H* and CH3O* (methoxy) were generated. This step is nearly thermoneutral (Er = −0.01 eV) with a 0.76 eV barrier. The breaking O−H length at the TS (TS3, Figure 7) is increased to 1.54 Å from 0.99 Å. After reaction, CH3O* is located on the Cu fcc hollow site, while H* prefers to stick to the Ni site. For C−H scission of methanol (R4), H* and CH2OH* were produced. This step has a 0.29 eV reaction energy and a barrier of 1.35 eV. At the TS (TS4, Figure 7), the breaking C−H length is elongated to 2.05 Å from 1.10 Å. Comparing with the reaction and barrier energies for R3 and R4, CH3O* formation (R3) was found to be more favored than CH2OH* formation (R4) over NiO/Cu(111), which is similar to that on Cu(111). Therefore, the reactivity of CH3O* produced by R3 on NiO/Cu(111) was only investigated in the following step. For CH3O* dissociation on NiO/Cu(111), two possible reactions including C−H cleavage (R5) and C−O cleavage (R6)

O−H scission (R3) and C−H scission (R4). For the R3 reaction, the O−H breaking of CH3OH* at the top site yields the species of H* and CH3O* coadsorbed at nearby fcc sites. In the TS (TS3, Figure 6), the O−H bond length increases from 0.99 to 1.61 Å. This step is almost thermoneutral (Er = −0.02 eV) and has 0.93 eV activation energy. In the case of R4, CH2OH* and H* were produced through C−H cleavage of CH3OH*. The bond length of breaking C−H at the TS (TS4, Figure 6) is elongated to 1.93 Å from 1.10 Å. After reaction, CH2OH* is located at a top site via the carbon atom, while H* is located at an fcc hollow site. The calculated reaction and activation energies for the step are 0.84 and 1.38 eV, respectively. By contrasting results of reaction energies and barriers in methanol decomposition (R3 and R4), CH3O* formation (R3) was found to be more favored. Therefore, the reactivity of CH3O* produced by R3 on Cu(111) was only considered in the following step. For the dissociation of CH3O* on Cu(111), there are two possible pathways: C−H bond cleavage (R5) and C−O bond scission (R6). For the R5 step, the formed CH2O* and H* adsorbed at top (C)−bridge (O) and fcc hollow sites, respectively. The C−H length in the TS (TS5, Figure 6) increases to 1.78 Å from 1.10 Å. The reaction has 0.79 eV reaction energy and a barrier of 1.18 eV. For the R6 reaction, corresponding products are CH3* and O*, which adsorbed on hcp and fcc sites, respectively. The C−O bond length in the TS (TS6, Figure 6) increases to 1.99 Å from 1.45 Å. This step has a reaction energy of 0.61 eV with an activation barrier of 1.62 eV. E


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Figure 5. Potential energy surfaces for MSR reaction on (a) Cu(111) and (b) NiO/Cu(111) surfaces. The // symbol denotes that H* and OH* species were removed from the next step of the calculation.

Figure 6. Structures of the optimized transition states for MSR reaction on the Cu(111) surface. The numeric values are the key bond distances for the elementary reactions involved. Atom color labeling: orange, Cu; gray, C; red, O; white, H.

length in the transition state (TS5, Figure 7) is increased to 1.83 Å from 1.10 Å. For C−O cleavage of CH3O* (R6), the C−O bond length at the transition state (TS6, Figure 7) increases to

are considered. For R5 reaction, the corresponding products are CH2O* and H*. This step possesses an activation barrier of 1.14 eV with 0.56 eV of reaction energy. The breaking C−H bond F


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Figure 7. Structures of the optimized transition states for MSR reaction on the NiO/Cu(111) surface. The numeric values are the key bond distances for the elementary reactions involved. Atom color labeling: orange, Cu; blue, Ni; gray, C; red, O; white, H.

2.04 Å from 1.10 Å. Then, the formed CH3* moves to the Cu hcp hollow site and O* adsorbs on the Ni site. The reaction energy and activation barrier for the step are 0.44 and 1.57 eV, respectively. By contrasting R5 and R6, CH2O* formation (R5) was calculated to be more favored than CH3* formation (R6) over NiO/Cu(111), which is similar to that on Cu(111). 3.2.2.3. Reactions of CH2O. On the Cu(111) surface, three competing reaction paths of CH2O* were considered. One is the dehydrogenation of CH2O* (R7), resulting in the formation of CHO* and H*. The bond length of C−H in the TS (TS7, Figure 6) is elongated to 1.68 Å from 1.10 Å. The step possesses activation and reaction energies of 0.74 and 0.18 eV, respectively. The second path is the dehydrogenation of CH2O* assisted with OH* (R8), resulting in the formation of CHO* and H2O*. The breaking C−H bond length in the transition state (TS8, Figure 6) is increased to 1.95 Å from 1.10 Å. This step has a reaction energy of 0.09 eV with an activation barrier of 0.66 eV. Then, CO* and H* were produced by the further dehydrogenation of CHO* (R9). The third pathway is the reaction of CH2O* and OH* (R10), resulting in the formation of CH2OOH*. The reaction and barrier energies are −0.46 and 0.15 eV, respectively. At the transition state (TS10, Figure 6), CH2 O* is elevated from the substrate for accommodating the OH* attacking. Comparing with R7, R8, and R10, it is found that R10 has the lowest barrier and the nature of exothermicity. Thus, CH2O* prefers to react with hydroxyl to produce CH2OOH* (R10), instead of dehydrogenation to CHO* (R7 and R8) on Cu(111). In the following reactions of CH2OOH*, two competing reactions (R11 and R12) were compared. It can be seen from Figure 5 and Table S4 (Supporting Information) that the reaction barrier of R12 is

lower than that of R11. Therefore, CH2OOH* dissociating to CHOOH* and H* (R12) is more favorable, and only the reactivity of CHOOH* is considered in the following step. The dehydrogenation of CHOOH* to CHOO** and H* (R13) on Cu(111) has an activation barrier of 0.51 eV and a reaction energy of −0.36 eV. The bond length of O−H at the transition state (TS13, Figure 6) is increased to 1.55 Å from 0.99 Å. Compared with the decomposition of CHOO* to CHO*, its dehydrogenation to CO2* was reported to have occurred much more quickly.51 Therefore, the dehydrogenation of CHOO* was only considered in this work. A two-stage process was contained in CHOO** dehydrogenation. First, CHOO** with bidentate adsorption structure converts into the unidentate CHOO* (R14, Ea = 0.53 eV). Second, CO2* and H* were formed through the C−H bond cleavage of unidentate CHOO* (R15), and this step exhibits a 0.51 eV activation energy. Thus, the overall barrier for these two steps (R14 + R15) is 1.04 eV, which is consistent with the theoretical 1.13 eV reported in the literature20 and compares with the experimental value of 1.10 ± 0.16 eV.52 On the NiO/Cu(111) surface, similar to Cu(111), three competing reactions of CH2O* were also compared. For CH2O* dehydrogenation (R7), CHO* and H* were produced. The bond length of C−H in the TS (TS7, Figure 7) is stretched to 1.64 Å from 1.10 Å. The reaction possesses a −0.51 eV reaction energy and an activation barrier of 0.20 eV. For CH2O* dehydrogenation with OH* assistance (R8), CHO* and H2O* were produced. At the TS (TS8, Figure 7), the breaking C−H length is elongated to 2.03 Å from 1.10 Å. The calculated reaction and activation energies for this step are −0.36 and 0.38 eV, respectively. For CH2O* reacting with OH* to CH2OOH* G


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Industrial & Engineering Chemistry Research (R10), the barrier and reaction energies are 0.49 and −0.25 eV, respectively. Comparing with R7, R8, and R10, it was found that R7 has the lowest barrier and the highest exothermicity, and thus, CH2O* dehydrogenation to CHO* and H* (R7) is most favorable on the NiO/Cu(111) surface. In the following, we only focus on the dehydrogenation of the resulting CHO* from R7. For the dehydrogenation of CHO* (R9), CO* and H* were produced. This reaction exhibits a 0.32 eV activation barrier and a −0.97 eV reaction energy. The C−H length in the transition state (TS9, Figure 7) is increased to 1.50 Å from 1.10 Å. 3.2.2.4. H2 Formation. On the Cu(111) surface, the coupling of hydrogen (R16) is endothermic (Er = 0.40 eV) with a 0.93 eV barrier, which is consistent with the available literature values (Er = 0.36 eV, Ea = 0.90 eV).28 In this step, the two H atoms located at nearby fcc sites moved to bridge sites via TS16 (Figure 6) and the H−H bond length is 1.01 Å. On the NiO/Cu(111) surface, the coupling of hydrogen (R16) is endothermic (Er = 0.33 eV) with a lower barrier of 0.81 eV, which is attributed to the interface of NiO/Cu(111).38 At the transition state (TS16, Figure 7), the H atoms are located at top sites and the bond length of H−H is 1.04 Å. 3.2.3. Brief Summary. On the basis of the study of elementary reactions, the preferred reaction pathways of MSR over Cu(111) and NiO/Cu(111) were summarized in Figure 8. It is found that

(CH3O*) dehydrogenation is the rate-limiting step, which is in good agreement with experiment results.53 On the NiO/Cu(111) surface, the reaction path for formaldehyde (CH2O*) formation from the CH3OH* was the same as that on Cu(111). Then, the following favorable step is CH2O* dehydrogenation to CHO*. Finally, further dehydrogenation of CHO* takes place and results in the formation of CO* and H*. Thus, CO and H2 are the main products on the NiO/Cu(111) surface for MSR. In addition, it was also found from Figure 8 that the difference of the MSR pathway between Cu(111) and NiO/Cu(111) comes from CH2O* reactivity. Over the Cu(111) surface, CH2O* preferentially reacts with hydroxyl from H2O* dissociation into CH2OOH*, and then, CH2OOH* further decomposes into H* and CO2*. However, over the NiO/ Cu(111) surface, CH2O* prefers to directly dehydrogenate into H* and CO*. The difference in CH2O* reactivities resulted in the distinct product selectivity for MSR over Cu(111) and NiO/ Cu(111). This provided further evidence on the experimental results and a fundamental understanding on the role of NiO species in affecting the mechanism of MSR reaction. 3.3. Reaction Pathways of DME SR over Supported Catalysts. According to the aforementioned analyses, the reaction mechanisms for diemthyl ether steam reforming on Cu/γ-Al2O3/Al and Cu/NiO/γ-Al2O3/Al bifunctional catalysts were summarized in Figure 9. For the overall DME SR process, hydrolysis of DME into MeOH on γ-Al2O3/Al is the first step, while two different pathways of the MSR step occurred over Cu and Cu/NiO catalysts, which resulted in different product distributions. One is methanol reacting with water, resulting in CO2 and H2 being the main products over Cu catalyst. The other is direct methanol decomposition to CO and H2 over Cu/NiO catalyst, which is similar to the mechanism over group 8−10 metals (metallic Ni, Pt, etc.). This indicates that existence of NiO in copper-based catalyst changed the MSR reaction pathway and promoted the decomposition of methanol, thus enhancing the formation of CO.

Figure 8. Preferred reaction pathways of MSR on Cu(111) and NiO/ Cu(111) surfaces.

the main reaction path on Cu(111) is as follows: First, methanol dehydrogenates to methoxy (CH3O*) via the bond cleavage of O−H and then C−H bond scission into formaldehyde (CH2O*). Second, CH2O* reacts with OH* (hydroxyl) produced through H 2 O* (water) dissociation into CH2OOH*. Finally, CH2OOH* dehydrogenates to formic acid (CHOOH*) through the bond scission of C−H, and then, CHOOH* completely dissociates into H* and CO2*. Therefore, CO2 and H2 are the main products on the Cu(111) surface for MSR. It was also found that the reaction of methoxy

4. CONCLUSIONS The catalytic performance of copper-based catalysts supported on monolithic anodic alumina in steam reforming of dimethyl ether was systematically discussed. The main conclusions of the work are summarized as follows:

Figure 9. Reaction mechanism of DME SR over different catalysts. H



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(1) For the DME SR process, hydrolysis of dimethyl ether to MeOH on Cu/γ-Al2O3/Al and Cu/Ni/γ-Al2O3/Al samples is the first step. For the MSR step, formation of H2 and CO2 via methanol reforming was dominant on copper-based catalyst, while MeOH decomposition to CO and H2 was the main reaction over Ni-doped Cubased catalyst, which is similar to that on metallic Ni. (2) It is evidenced that nickel species existed in the form of an oxidation state (NiO) on the Cu/Ni/γ-Al2O3/Al surface during reaction. DFT calculations were further conducted to investigate reaction paths of MSR over Cu(111) and NiO/Cu(111). The preferred reaction route on Cu(111) is shown below: methanol → methoxy → formaldehyde → hydroxymethoxy → formic acid → formate → carbon dioxide, while the main reaction path on NiO/Cu(111) is shown below: methanol → methoxy → formaldehyde → formyl → carbon monoxide. The proposed pathways agree well with the experiment results in this work and provide a further theoretical explanation for the phenomenon. (3) A similar reaction path for CH2O* formation from CH3OH* through first O−H cleavage and then C−H cleavage was identified over Cu(111) and NiO/Cu(111) surfaces. Moreover, CH2O* has a significant role in the MSR reaction. The different CH2O* reactivities on Cu(111) and NiO/Cu(111) resulted in the distinct product selectivity. On the Cu(111) surface, CH2O* preferentially reacts with OH* to produce CH2OOH*. Then, CH2OOH* decomposes to CO2* and H*, while CH2O* tends to dehydrogenate to H* and CO* directly on NiO/Cu(111), which provides significant insights into reaction mechanisms and is useful for rational design of durable and selective Cu-based catalysts.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.8b02628. Summaries for reaction mechanism of MSR based on DFT calculations (Table S1); comparison of the calculated adsorption energies for MSR intermediates in this work with previous experimental and theoretical values on the Cu(111) surface (Table S2); adsorption energies and configurations of MSR intermediates on Cu(111) and NiO/Cu(111) surfaces (Table S3); calculated activation barriers (Ea) and reaction energies (Er) for elementary reactions on Cu(111) and NiO/ Cu(111) surfaces (Table S4) (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Feiyue Fan: 0000-0002-9614-0299 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the financial support from the Shanghai Rising-Star Program (B type) (No. 13QB1401300). I


Article

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Insights into the CO Formation Mechanism during Steam Reforming of

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