Reactivity of Methanol Steam Reforming on ZnPd Intermetallic Catalyst

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Reactivity of Methanol Steam Reforming on ZnPd Intermetallic Catalyst: Understanding from Microcalorimetric and FT-IR Studies Xiaoyu Li, Lin Li, Jian Lin, Botao Qiao, Xiaofeng Yang, Aiqin Wang, and Xiaodong Wang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b03933 • Publication Date (Web): 21 May 2018 Downloaded from http://pubs.acs.org on May 21, 2018

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The Journal of Physical Chemistry

Reactivity of Methanol Steam Reforming on ZnPd Intermetallic Catalyst: Understanding From Microcalorimetric and FT-IR Studies

Xiaoyu Lia,b, Lin Lia,*, Jian Lina, Botao Qiaoa, Xiaofeng Yanga, Aiqin Wanga, Xiaodong Wanga,* a

State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese

Academy of Sciences, Dalian 116023, China. b

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

*Corresponding author. Tel: +86-411-86379680; fax: +86-411-84691570 E-mail address: [email protected] *Corresponding author. Tel:＋86-411-84379677; fax: +86-411-84685940 E-mail address: [email protected]

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Abstract Methanol steaming reforming (MSR), catalyzed by Pd/ZnO, is a promising process to produce onboard hydrogen for fuel cell. The reactivity of Pd/ZnO, especially selectivity to CO2 and H2, changes with the formation of ZnPd intermetallic compound and ZnPd-ZnO interface. In this work, we measured the adsorption energetics and natures of adsorbed species on ZnO, Pd/ZnO, and ZnPd/ZnO catalysts by combining adsorption microcalorimetry and infrared spectroscopy with the reactants (methanol and water) and intermediate (formaldehyde) of MSR as probe molecules, and correlated the adsorption energetics to the reactivities of the samples. ZnO exhibits weakly molecular adsorption for methanol while strongly interacting with water. In contrast, the adsorption energetic gap between methanol and water decreases on Pd/ZnO and disappears on ZnPd/ZnO. This might be responsible for the highest activity of MSR on ZnPd/ZnO since methanol could competitively adsorb and react with water. Though the introduction of Pd onto ZnO lowers the thermodynamic stability of adsorbed formaldehyde, the formed ZnPd intermetallic compound strengthens the bonding of adsorbed formaldehyde, allowing the further reaction with water that results in the progress of the reaction pathway to CO2 and H2. This might be an important factor for the high selectivity to CO2 and H2 on ZnPd/ZnO.

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1. Introduction Hydrogen, as a promising alternative fuel, has been used to produce electrical power in fuel cells with improved energy efficiency and less pollution. A main obstacle for fuel cell applications is the difficulties associated with the storage and transportation of hydrogen. Methanol steam reforming (CH3OH + H2O →3 H2 + CO2) reaction has provoked tremendous research activities since it can produce high purity hydrogen on board for fuel cell applications.1-2 The suppression of CO, a by-product of MSR, is the greatest demand since CO can poison the anode of fuel cells severely.3 Conventional Cu-based catalysts catalyze MSR with high activity and selectivity to H2 and CO2; however, they are pyrophoric once reduced and prone to sintering even at moderate temperatures (>553 K).1,

4

To overcome the drawbacks of Cu-based

catalysts, many other catalyst formulations were developed, among which the ZnO supported Pd catalyst have attracted considerable attention due to their high activity, long-term stability as well as excellent selectivity to CO2 and H2.5-6 With extensive studies, a consensus has now been achieved that the high activity and selectivity to CO2 on Pd/ZnO are only obtained after the formation of ZnPd intermetallic compound under reaction conditions or upon high-temperature reduction.7-8 That is, the formation of ZnPd intermetallic compound is critical to the high activity and selectivity. Besides the ZnPd intermetallic phase, the ZnO support and/or oxidized Zn also have been recently demonstrated to play an important role in the activity and selectivity of ZnPd catalyst.8-9 Indeed, ZnO itself has been identified 3



as an highly selective catalyst for CO2 in MSR.10-11 A teamwork between ZnO and ZnPd has been proposed to be responsible for increasing CO2 selectivity through the interface between ZnPd and ZnO, which promote the activation of water.8 Furthermore, based on density functional theory calculations (DFT) and X-ray photoelectron spectroscopy (XPS), a modification of electronic property was found with the formation of ZnPd intermetallic compound.12 The special electronic property, as well as the interface between ZnPd and ZnO was suggested as the cause of catalytic reactivity by altering the bonding strengths of reactants or intermediates. Nevertheless, the direct correlation between the adsorption energetics of reactants or intermediates and the reactivity on Pd/ZnO is still missing and remains to be explored. Although the exact mechanism of MSR catalyze by Pd/ZnO has not been fully understood yet, it is generally accepted that MSR is initiated with the dehydrogenation of methanol to produce methoxyl (CH3O), and further dehydrogenation to formaldehyde (HCHO).6, 13 The adsorbed formaldehyde has been considered as the key intermediate of MSR reaction, which has been validated by the almost same products of formaldehyde steam reforming as that of MSR.14 The formed formaldehyde can serve as branching point for different products. The reaction of formaldehyde with OH/H2O was suggested as the key step for the CO2 selectivity on ZnPd while the dehydrogenation of formaldehyde to CO was favored on Pd.2, 7, 13-16 Nevertheless, the underlying reasons for these different reaction routes are under debate. Takezawa and coworkers17 speculated that ZnPd alloy formation could alter the bonding configuration of adsorbed formaldehyde intermediates, lowering the 4


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adsorption energies, and therefore avoiding the further dehydrogenation of formaldehyde. DFT calculations confirmed that formaldehyde adsorbed on Pd(111) and PdZn(111) with adsorption energies of -43 and -23 kJ/mol, respectively.18 Also, DFT studies indicated that PdZn(111) exhibited the weakest adsorption for formaldehyde with adsorption energies of -25 ~ -18 kJ/mol, as compared to NiZn(111) and PtZn(111) in the range of -60 ~ -37 kJ/mol and -45 ~ -17 kJ/mol, respectively.16 A correlation between weak adsorption energetics of formaldehyde and CO2 selectivity has been proposed.16 In contrast, temperature programmed desorption (TPD) studies on Zn/Pd(111) indicated that the addition of 0.03 ML of Zn on Pd(111) produced two desorption peaks of formaldehyde at 210 and 360 K, respectively, suggesting that the stability of adsorbed formaldehyde increased.19 A TPD study revealed that the calculated desorption energies on the unsupported Pd, PdZnα and PdZnβ1 were 122, 152 and 164 kJ/mol, respectively. Higher adsorption strengths existed on the intermetallic compounds.20 These studies pointed out the important role of adsorption energetics of formaldehyde in determining the CO2 and H2 selectivity. Nevertheless, considering these studies were mainly based on the model systems and theoretical calculations without the involvement of ZnPd-ZnO interface, a direct measurement of adsorption energetics of formaldehyde is highly desire and valuable to clarify the reactivity of MSR on actual ZnPd catalyst. In this work, the adsorption behaviors of both reactants of methanol and water and intermediate

of

formaldehyde

were

systematically

studied

by

means

of

microcalorimetry, a powerful tool to determine the bonding strengths of adsorbed 5



species on catalyst surface,21-23 and in situ FT-IR. A good correlation of the adsorption energetics to the reactivity including both activity and selectivity to CO2 on the samples was established. This study not only experimentally resolved the argue on adsorption of intermediate on Pd/ZnO and ZnPd/ZnO catalysts, but also provided new insights into the reactivity of ZnPd catalyst from adsorption energetic point of view thus will contribute new understanding to this field.

2. Experimental 2.1. Catalyst preparation All chemicals were analytical grade and used without further purification. ZnO (BET surface area of 37.2 m2 g-1) was purchased from Shanxi Sino-Academy Nano-Materials Co., Ltd. The nominal Pd loading of Pd/ZnO is 1 wt%. ZnO supported Pd catalysts were prepared by the incipient impregnation method with dilute aqueous solution of palladium nitrate as reported previously.24 After impregnation at room temperature, the samples were dried at 393 K for 12 h and calcined at 673 K for 3 h. 2.2. Catalyst characterization The Pd loading of the as-prepared sample was determined by inductively coupled plasma spectrometry (ICP-AES) on an IRIS Intrepid II XSP instrument (Thermo Electron Corporation). The sample was dissolved into the nitro hydrochloric acid. The PW3040/60 X’Pert PRO (PANalytical) diffractometer with a copper anode (Cu Kα, λ = 0.15432 nm) was employed to record X-ray diffraction (XRD) patterns, 6


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operating at 40 kV and 40 mA. Data collection was under the same conditions for all samples with the scanning angel (2θ) from 10 º to 80 º with the speed of 3 º min-1. The H2 temperature-programmed reduction (H2-TPR) was carried out on a Micromeritics AutoChemⅡ2920 instrument. Prior to the H2-TPR measurement, the as-prepared sample (100 mg) was put in a U-shaped quartz reactor and pretreated in Ar with a flow rate of 30 ml min-1 at 473 K for 30 min, then cooled down to 223 K and kept until the baseline was stable. The sample was heated from 223 to 873 K with the heating rate of 10 K min-1 in a flowing gas of 10 vol% H2/Ar. Based on the H2 peak area detected by thermal conductivity detector (TCD) and calibration curve of the 10 vol% H2/Ar standard gas, H2 consumption amount was calculated. Microcalorimetric measurements of CH3OH, HCHO and H2O adsorption were performed using a Calvet-type heat-flux calorimeter (Setaram BT 2.15, France). The pretreatment and subsequent adsorption measurement were performed in a home-made quartz treatment cell. The sample of ca. 125 mg was loaded in the cell and then was first calcined at 673 K with air to remove the adsorbed species on the surface. The Pd/ZnO and ZnPd/ZnO were obtained by further reduced with H2 for 1 h at 373 and 673 K, respectively. Then, Pd/ZnO and ZnPd/ZnO were in situ outgassed in vacuum at the reduction temperature for 3 h, and ZnO sample was evacuated at 673 K for 3 h. Following pretreatment of samples, the cell was isolated and transferred into microcalorimeter, and then evacuated to a vacuum of ca. 10-4 Pa. A stable baseline heat signal was attained after approximately 6-8 h thermal equilibrium between the cell and the microcalorimeter. The adsorption experiment was conducted 7



at 313 K. Methanol (99.9%, HPLC) and ultrapure water, whose vapors used for adsorption studies, were purified by successive freeze/pump/thaw cycles with liquid nitrogen. The gaseous formaldehyde was obtained by the saturated vapor of polyformaldehyde at 323 K. The microcalorimetric data were collected by successively introducing small doses (1 ~ 10 µmol) of probe molecules onto the sample until it became saturated with a equibrium pressure of 5 ~ 6 Torr. In a typical microcalorimetric experiment, the plots of differential heat versus adsorbate uptake and adsorption isothermals can be obtained simultaneously. In situ Fourier transform infrared (FTIR) spectra of CO adsorption on various samples were collected on a VERTEX 70V infrared spectrometer equipped with a mercury cadmium telluride (MCT) detector at a resolution of 4 cm-1. The experiments were carried out in a high temperature reaction chamber (HVC-DRP-5, Harrick) equipped with ZnSe windows. Prior to CO adsorption, the samples were pretreated with air at 673 K and in situ reduced by H2 to prepare Pd/ZnO (at 373 K) and ZnPd/ZnO (at 673 K), followed by purging with He at the same temperature for 30 min. After the temperature decreased to room temperature, the background spectrum was collected. Then the spectrum of CO adsorption were recorded with ca. 3 Torr CO introduced. In situ diffuse reflectance infrared Fourier transform (DRIFT) spectra of CH3OH, HCHO and H2O were acquired with the VERTEX 70V infrared spectrometer. After the same sample pretreatments as microcalorimetric experiments, the vapor of methanol and water were introduced onto the samples using a liquid bubbler system 8


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with He as a carrier gas, and gaseous formaldehyde is the 40 ppm formaldehyde/He mixture purchased from Dalian GuangMing special gas co. LTD. The adsorption amount of probe molecules was controlled by adjusting purge time of probe molecules with a total flow of 20 mL min-1. 2.3. Catalyst test The methanol stream reforming (MSR) reaction on Pd/ZnO, ZnPd/ZnO, and ZnO was tested in a continuous U-shaped fixed-bed quartz tubular reactor from 523 to 673 K under atmospheric pressure with 50 mg of a catalyst in about 20-40 mesh size. The ZnO supported Pd catalysts were purged with He for 1 h to remove the adsorbed hydrogen on the surface after the in situ reduction. The CH3OH and ultrapure water were premixed in an airtight bottle to prevent volatilization. Then the mixture was pumped by MasterFlex model 77390-00 Cole-Parmer Teflon Tubing Pump, vaporized at 393 K and carried out by He. The feedstock gas for methanol steam reforming consists of 8 vol% CH3OH, 12 vol% H2O and He balance. The total gas flow rate was 46 mL min-1, which resulted in a space velocity of 55200 mL h-1g-1cat. The inlet and outlet gas composition were analyzed on-line by Agilent 6890A gas chromatograph with a HS-D 100/120 column and thermal conductivity detector (TCD). H2, CO, and CO2 were the only detected products. The methanol conversion (X) in the MSR reaction and CO2 selectivity (S) were calculated as follows.8

X=

݊ሺCOሻ + nሺCOଶ ሻ × 100% n’ሺCHଷ OHሻ

9



S=

nሺCOଶ ሻ × 100% ݊ሺCOሻ + nሺCOଶ ሻ

Wherein n(CO2) and n(CO) are the molar concentration of CO2, CO in the product gas, n’(CH3OH) are the molar concentration of CH3OH in the feed gas.

3. Results and discussion 3.1 Catalyst characterization The Pd metal loading on the as-prepared Pd/ZnO is 0.94 wt% determined by ICP-AES. H2-TPR measurement was performed to determine the reduction performance of as-prepared Pd/ZnO and the result was presented in Figure 1. A sharp reduction peak located at 315.7 K with a H2 consumption amount of 301.4 µmol/g was observed. This value is much higher than the theoretical value of the reduction of PdO species to metallic Pd (81.7 µmol/g), suggesting the formation of palladium hydride or H2-spillover through strong metal-support interaction (SMSI).6, 25-26

After that, the slow increase of TCD signal was observed from 400 K and two

broad peaks appeared at 506.7 and 756.8 K, respectively, suggesting that the formation of ZnPd intermetallic compound occurred at > 400 K and in a quite slow process.27 According to this H2-TPR result and our previous reports,24, 28 the ZnO supported Pd catalyst (denoted as Pd/ZnO) in this work was just reduced at a relatively low temperature of 373 K to avoid the formation of intermetallic compound while the ZnO supported ZnPd intermetallic catalyst (denoted as ZnPd/ZnO) was reduced at 673 K to ensure the formation of ZnPd intermetallic 10


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compound.

Figure 1. TPR profiles of as-prepared Pd/ZnO.

XRD measurements were performed to detect the crystalline phases of various samples. As shown in Figure 2, the well-resolved reflection peaks on the three samples indicated by asterisks are assigned to wuritze-type phases of ZnO. On all samples the peak ascribed to ZnO phase did not shift, indicating that ZnO phase is well maintained. No reflection peaks of Pd was observed on Pd/ZnO and ZnPd/ZnO, indicating the good dispersion of Pd. Instead, a weak diffraction peak situated at 2θ=41.2º and 44.1º, attributed to ZnPd (111) and ZnPd (200) intermetallic phase29, as expected, observed on ZnPd/ZnO, indicating the formation of ZnPd crystalline phase.

11



Figure 2. XRD patterns of ZnO, Pd/ZnO and ZnPd/ZnO samples.

The XRD is a bulk technique and could not probe the surface structures of various sample. Thus, the CO adsorption FT-IR was used to probe the surface Pd sites, and the corresponding results are shown in Figure 3. We found that no obvious CO adsorption peaks were produced on ZnO when CO was introduced. In contrast, two CO adsorption peaks were observed on Pd/ZnO. The peak at 2084 cm-1 is assigned to the linear on-top CO adsorption on metallic Pd. The broad peak centered at 1965 cm-1 is attributed to bridged and multi-bonded CO adsorption. The linear species on ZnPd/ZnO is located at 2068 cm-1, which is 16 cm-1 red-shift as compared to that on Pd/ZnO. The red-shift reflects the electron transfer from Zn to Pd,30 which resulted in an increase of back donation of electrons from the Pd into the CO 2π* antibonding orbital. This provides another evidence for the formation of ZnPd intermetallic phase. Furthermore, the geometric modification of Pd with the formation of Pd-Zn-Pd ensembles would weaken/eliminate the bridged and multi-bonded CO species. As expected, the peaks of bridged and multi-bonded CO decreased significantly though 12


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a weak peak was still observed at 1974 cm-1, which was also observed by other researchers and was attributed to the surface change induced by CO exposure.31 The change in the geometric and electronic effects with the formation of ZnPd intermetallic compound suggests that ZnPd/ZnO would exhibit different adsorption and activiation for the reactants and intermediate of MSR.

Figure 3. FT-IR of CO adsorption on Pd/ZnO and ZnPd/ZnO.

3.2 Catalytic activity test Methanol conversion and CO2 selectivity versus reaction temperature from 523 to 673 K on ZnO, Pd/ZnO, and ZnPd/ZnO in MSR are illustrated in Figure 4. Initially, the activity of MSR on ZnO was very low and the apparent conversion of methanol was only observed from about 600 K. Nevertheless, the selectivity to CO2 exceeded 95% and decreased slightly with reaction temperature. This is in agreement with the previous reports that ZnO exhibited high CO2 selective in MSR.10-11 In comparison with ZnO, Pd/ZnO exhibited much higher MSR activity but with a much lower 13



selectivity to CO2. The selectivity increased with the reaction temperature, which was suggested to originate from the formation of ZnPd intermetallic phase under reaction condition.7 In particular, ZnPd/ZnO exhibited not only an even higher activity but also a much higher CO2 selectivity as compared with Pd/ZnO. Obviously, the formation of ZnPd intermetallic phase and ZnPd-ZnO interface is favorable to the high performance in MSR.

Figure 4. Methanol conversion (open symbols) and CO2 selectivity (solid symbols) as a function of reaction temperature over ZnO (), Pd/ZnO () and ZnPd/ZnO () in MSR. Reactant mixture: CH3OH (8 vol%), H2O (12 vol%), He (Balance). The total follow is 46 mL min-1 and GHSV=55200 mL h-1g-1cat.

3.3 Microcalorimetric Studies To study the adsorption behaviors of various reactants and intermediates of MSR, we first employed adsorption microcalorimetry to study their adsorption strengths. And then, the FT-IR measurements were performed to probe the nature of adsorption 14


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species in the corresponding microcalorimetric experiments. Table 1 shows the initial heats and saturation uptakes for methanol, water, and formaldehyde on ZnO, Pd/ZnO, and ZnPd/ZnO samples.

Table 1. Initial heats and saturation uptakes for methanol, water, and formaldehyde adsorption on ZnO, Pd/ZnO and ZnPd/ZnO at 313 K.

Methanol

Water

Formaldehyde

Heat

Uptakea

Heat

Uptakea

Heat

Uptakea

(kJ/mol)

(µmol/g)

(kJ/mol)

(µmol/g)

(kJ/mol)

(µmol/g)

ZnO

61

207

174

248

155

351

Pd/ZnO

73

141

96

24

109

169

ZnPd/ZnO

144

143

145

139

145

108

Samples

a

: refer to the amount with the adsorption heat higher than 40 kJ/mol

3.3.1 Methanol adsorption The dissociative adsorption of methanol is the starting point of the MSR reaction.6 As shown in Table 1 and Figure 5, methanol adsorption on ZnO produced an initial heat of 61 kJ/mol and this differential heat remained constant within the first 140 µmol/g of uptake. After that, the differential heat decreased slowly until a saturation uptake of 207 µmol/g was reached. On the contrary, Pd/ZnO had a slightly higher initial heat of 75 kJ/mol and ZnPd/ZnO generated a much higher initial heat of 144 kJ/mol, although the saturation uptakes on them are lower than that on ZnO 15



support. TPD studies of unsupported Pd and PdZn also demonstrated a higher desorption energies of methanol on PdZn with the calculated energies of 115, 122, and 134 kJ/mol on Pd, PdZnα and PdZnβ1.20

Figure 5. Differential heat versus uptake for the adsorption of methanol on ZnO, Pd/ZnO and ZnPd/ZnO at 313 K.

In order to understand the different adsorption heat, i.e., the interaction strengths between methanol and various samples, DRIFT measurements were performed to probe the adsorption species of methanol. The corresponding results are shown in Figure 6. According to the literature,32-36 the 2946, 2829, 1454, and 1052 cm-1 bands were ascribed to the methoxy species, which correspond to the vibrations of νas(CH3), νs(CH3), δ(CH3), and ν(C-O), respectively. The negligible absorption peaks associated with methoxy species on ZnO suggested that methanol mainly adsorbs on ZnO molecularly. Previous studies also indicated that ZnO powder adsorbs methanol physically at low temperatures.37 Based on FT-IR results, the initial heat of 61 kJ/mol 16


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measured in microcalorimetric experiment could be attributed to the molecular adsorption heat of methanol, which is in consistent with the value of 60.5 ± 0.8 kJ/mol on Pt(111) measured by single-crystal adsorption calorimetry (SCAC).38 Adsorption peaks associated with methoxy species were clearly observed on Pd/ZnO catalyst, indicating that methanol adsorbed dissociatively. Campbell and coworkers38 have determined the bond strength of adsorbed methoxy with Pt(111) to be 187±11 kJ/mol calorimetrically, which was much higher than the molecular adsorption heat. Thus, the combination of FT-IR and microcalorimetry results, we can conclude that dissociative adsorption of methanol has occurred although methanol is still mainly molecular adsorption on Pd/ZnO. Obviously, introduction of Pd can promote the dissociation of methanol, which is beneficial for the MSR reaction. As compared with that on Pd/ZnO, a much higher intensity of methoxy peaks was observed on ZnPd/ZnO, suggesting that the formation of ZnPd intermetallic compound and ZnPd-ZnO interface significantly promotes the dissociation of methanol, leading to the highest adsorption heat of 144 kJ/mol among the three samples.

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Figure 6. In situ DRIFT spectra of methanol adsorption on (a) ZnO, (b) Pd/ZnO and (c) ZnPd/ZnO catalysts.

3.3.2 Water adsorption

Figure 7. Differential heat versus uptake for the adsorption of water on ZnO, Pd/ZnO and ZnPd/ZnO at 313 K.

H2O, the other reactant in MSR, has been proposed to react with formaldehyde to form the species such as hydroxymethoxy, formic acid or formate, which further decompose to CO2 and H2.14-15,

17, 39

As shown in Table 1 and Figure 7, water

adsorption on ZnO produced an initial heat of 174 kJ/mol, which is slightly higher that the values of 16040 and 170 kJ/mol41 reported previously on other ZnO samples. The differential heat decreased slowly with water uptake until a saturation uptake of 248 µmol/g. Different from those of methanol adsorption, the initial heat and saturation uptake of water adsorption decreased to 96 kJ/mol and 24 µmol/g, 18


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respectively, after introduction of Pd species. In contrast, ZnPd/ZnO exhibited moderate strength of 145 kJ/mol and saturation uptake of 139 µmol/g among the three samples.

Figure 8. In situ DRIFT spectra of water adsorption on (a) ZnO, (b) Pd/ZnO and (c) ZnPd/ZnO.

Figure 8 shows the DRIFT spectra of water adsorption on ZnO, Pd/ZnO and ZnPd/ZnO catalysts. When ZnO was exposed to water, the obvious hydroxyl bands which result from the molecular adsorption and dissociation of H2O molecules could be observed. Based on the FT-IR studies together with a detailed assignment of the vibrational bands,42-43 the broad peak in the range of 3700-2800 cm-1 and at 1643 cm-1 on ZnO originates from the coexistence of various OH species and molecularly adsorbed H2O. DFT calculation and TPD studies on ZnO single crystal indicated that the desorption of molecularly adsorbed H2O occurred at relative low temperature of 370 K, corresponding to desorption energy of 99 kJ/mol. The desorption hydroxyl 19



Page 20 of 36

species is associated with a broad peak between 320 and 550 K.44-45 The appearance of hydroxyl bands with high intensities can be correlated to the highest adsorption strength and uptake on ZnO, suggesting that water could be activated by ZnO easily. In comparison, the intensities of OH/H2O bands on Pd/ZnO were reduced significantly with the introduction of Pd. According to previous studies,42-43 the adsorption and dissociation of water usually occurs on structural defects or polar sites of ZnO. The significant decrease in the OH/H2O intensities suggests that the supported

Pd

species

occupied

these

sites,

and

in

turn

decreased

the

adsorption/dissociation of water, leading to much lower adsorption energetics of 96 kJ/mol on Pd/ZnO. Surprisingly, ZnPd/ZnO exhibited higher intensities of OH/H2O bands than Pd/ZnO when considering that ZnPd species could also occupy the same sites as Pd. Friedrich et. al proposed that the interface between ZnPd and ZnO patches improves the ability of ZnPd/ZnO to activate water.8 Thus, the higher adsorption energetics of 145 kJ/mol on ZnPd/ZnO as compared with Pd/ZnO demonstrates that the interface between ZnPd and ZnO is beneficial for the activation of water. .

3.3.3 Formaldehyde adsorption Formaldehyde is considered as an important intermediate involved in the MSR reaction. In order better to present the adsorption energetic distribution, the histograms of differential heats for formaldehyde adsorption on ZnO, Pd/ZnO, and ZnPd/ZnO are shown in Figure 9. These histograms were produced by smoothing the differential heat data of Figure S1 in supporting information with a 20


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least-squares-fitted polynomial to determine the number of active sites within a given range of differential heats.46 As shown in Table 1 and Figure 9, the initial heat of formaldehyde adsorption on ZnO was 155 kJ/mol. ZnO contains the largest number of sites for formaldehyde adsorption, and majority of the adsorbed formaldehyde interact with sites to produce differential heat lower than 110 kJ/mol. Nevertheless, there exists some sites interact with formaldehyde with adsorption energetics higher than 110 kJ/mol. It is worthwhile to note that these sites interacting strongly with formaldehyde disappeared completely on Pd/ZnO, yielding an initial heat of only 109 kJ/mol. In comparison, part of these sites was remained on ZnPd/ZnO, and the initial adsorption heat of formaldehyde only decreased slightly to 145 kJ/mol. Obviously, ZnPd/ZnO keeps part of the strong sites for formaldehyde adsorption while Pd removes these sites totally. Several TPD studies have revealed that the desorption temperature of formaldehyde is higher on PdZn as compared with Pd19-20. Especially, the desorption energetics on unsupported Pd, PdZnα and PdZnβ1 were 122, 152 and 164 kJ/mol, respectively,20 which is comparable to our values.

21



Figure 9. Differential heat versus uptake for the adsorption of water on ZnO, Pd/ZnO and ZnPd/ZnO at 313 K.

The DRIFT spectra of formaldehyde adsorption on ZnO, Pd/ZnO, and ZnPd/ZnO are illustrated in Figure 10. After the ZnO was exposed to HCHO/He mixture, several absorption bands were obviously observed. According to the previous studies,47-51 the bands at 2947, 2877 and 2763 cm-1 were assigned to ν(C-H) stretch vibration of formate species, and the bands at 1578 and 1369 cm-1 correspond to the νas(COO) and νs(COO) vibrations of formate species. In addition, the broad band at 1722 cm-1 could be ascribed to the molecularly adsorbed HCHO47 and the bands at 1295 and 1105 cm-1 are assigned to the weakly adsorbed polyoxymethylene.51 Among various adsorption species, the adsorbed formates exhibited highly thermodynamic stability.51 and the bond strength of adsorbed formates on Pt(111) has been determined calorimetrically to exceed 200 kJ/mol.52 Thus, the initial adsorption energetics of 155 kJ/mol on ZnO could originate mainly from the formation of adsorbed formate species. As compared with those on ZnO, nearly all adsorption bands on Pd/ZnO decreased, even disappeared, especially those of formates, which was in agreement with the microcalorimetric results that all the strong adsorption sites have been removed totally. It is worthwhile to note that the change tendency of adsorption heat of formaldehyde with Pd introduction on ZnO is in consistent with that of water, suggesting that the adsorption sites for formaldehyde and water could be the same. In contrast, ν(C-H) stretch vibration bands of formate species could still be observed on ZnPd/ZnO, in 22


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agreement with the existence of some strong adsorption sites in the microcalorimetric measurements, demonstrating the ability for the activation of formaldehyde.

Figure 10. In situ DRIFT spectra of formaldehyde adsorption on (a) ZnO, (b) Pd/ZnO and (c) ZnPd/ZnO.

3.4 Correlating the adsorption energetics to the activity and selectivity of MSR A chemical interaction of reactive species with catalyst surface occurs thoroughly in the catalytic cycle. The species interacting with catalyst too strong will cover the solid surface and in turn inhibit the adsorption and subsequent reaction of the other species. In contrast, if one species interacts with catalyst surface too weakly to be activated, the reaction can occur neither. The measurement of the interaction strengths is therefore of importance to understand the catalytic reactivity. As for the adsorption of reactants of MSR on ZnO, the results of microcalorimetry and FT-IR indicated that methanol adsorbed molecularly on ZnO with adsorption energetics of 61 kJ/mol while ZnO activated water with adsorption strength of 174 23



kJ/mol. This significantly energetic gap between methanol and water implies that adsorbed OH/H2O species would cover the surface of ZnO and inhibit the adsorption and subsequent reaction of methanol, leading to low activity for MSR at low temperatures. Increasing the reaction temperature is efficient to promote the progress of reaction since some weakly adsorbed OH/H2O would desorb from the surface; on the other hand, high temperature also facilitate the dissociative adsorption of methanol with the formation of methoxy.53 As a result, ZnO only exhibited the MSR activity from a relatively high temperature of 600 K. In comparison, the adsorption energetics of water decreased from 174 kJ/mol to 96 kJ/mol, and the initial heat of methanol increased from 61 kJ/mol to 73 kJ/mol on Pd/ZnO, implying that the energetic gap between these two reactants reduces significantly. The lower energetic gap means that adsorption of methanol could compete with water, allowing the occurrence of MSR at a lower reaction temperature of 525 K. It is noteworthy that the energetic gap between water and methanol disappeared totally when ZnPd intermetallic compound was formed (Table 1). The almost same adsorption energetics of water and methanol implies that these species competitively adsorb and subsequently react on ZnPd/ZnO catalyst, leading to an even higher MSR activity than Pd/ZnO (Figure 4). The selectivity of MSR can also be understood in terms of the adsorption measurement results. According to the DFT studies of MSR, the adsorbed formaldehyde either decompose directly to CO and H2, or react with OH/H2O to form the species such as hydroxymethoxy, formic acid, or formates, and further decompose 24


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to CO2 and H2. Thus, the selectivity to CO2 and H2 is dependent on the adsorption and activation of both water and formaldehyde. The results of microcalorimetry and FT-IR indicated that ZnO possessed the sites interacting strongly with water and formaldehyde. However, the introduction of Pd might occupy these sites, leading to the significant decrease of adsorption energetics. Nevertheless, the formed ZnPd intermetallic compound can adsorb and activate water and formaldehyde with moderate strength. Based on these results, it is very valuable to understand the selectivity of the three samples according to the adsorption energetics of water and formaldehyde. In general, the desorption temperature (K) of adsorbed species is approximately equal to the four times of adsorption energetics in a unit of kJ mol-1.54-55 According to the initial heats, the desorption temperatures of OH/H2O on ZnO, Pd/ZnO, and ZnPd/ZnO were calculated to be 696, 384 and 580 K, respectively. The corresponding values of adsorbed formaldehyde are 620, 436 and 580 K, respectively. The desorption temperatures of water and formaldehyde on Pd/ZnO are lower than the reaction temperature of MSR (520 K, Figure 4), suggesting that the reaction route from formaldehyde and water to other species could be suppressed significantly since both of them could not bind strongly enough on catalyst surface to react, instead of desorbing or decomposing directly. In contrast, the higher desorption temperatures of water and formaldehyde on ZnO and ZnPd/ZnO imply that the pathway of the reaction of formaldehyde and OH/H2O could proceed smoothly, resulting in the high selectivity to CO2 and H2 (Figure 4). It is also noteworthy that ZnPd/ZnO exhibited comparable adsorption strengths of 25



ca. 145 kJ/mol for reactants and intermediate, suggesting the competitive adsorption and reaction of all the species in the course of reaction. The desorption temperatures of these species are calculated to be 580 K, which fall in the reaction temperature range, ensuring the high MSR activity and selectivity to CO2. When the reaction temperature exceeds the desorption temperature, the selectivity decreases slightly since part of formaldehyde and H2O/OH could desorb although the activity increases correspondingly (Figure 4).

4. Conclusions The adsorption energetics of surface species of reactants (methanol and water) and intermediate (formaldehyde) of MSR on ZnO, as well as on ZnO supported Pd and ZnPd catalysts can be correlated to their catalytic reactivities. As for the adsorption of reactants, significant energetic gap between H2O with initial heat of 174 kJ/mol and methanol with initial heat of 61 kJ/mol exists on ZnO. Water might cover the surface of ZnO and inhibit the adsorption and subsequent reaction of methanol during MSR reaction. The introduction of Pd decreases the energetic gap by increasing the adsorption heat of methanol to 73 kJ/mol while decreasing the adsorption energetics of water to 96 kJ/mol, which allows the occurrence of the MSR reaction at lower temperature. The energetic gap disappears totally on ZnPd/ZnO with almost same energetics of ca. 145 kJ/mol, leading to the highest MSR activity because of the competitive adsorption and subsequent reaction between reactants. As for the intermediate adsorption, ZnO exhibits an initial heat of 26


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155 kJ/mol for the adsorption of formaldehyde. The introduction of Pd decreases significantly the interaction strength to 109 kJ/mol while the formed ZnPd intermetallic compound exhibits a moderate adsorption strength of 145 kJ/mol for formaldehyde. As a result, the reaction between adsorbed formaldehyde and water with moderate and comparable strength is expected to occur on ZnPd/ZnO, accounting for its high selectivity to CO2 and H2.

Supporting Information The plot of differential heat versus uptake for formaldehyde adsorption. This material is available free of charge via the Internet at http://pubs.acs.org.

Author information *Corresponding author Xiaodong Wang: Tel:

+86-411-86379680;

fax:

+86-411-84691570;

E-mail

address:

[email protected] Lin Li: Tel:＋86-411-84379677; fax: +86-411-84685940; E-mail address: [email protected]

ORCID Lin Li: 0000-0002-3036-0934 Botao Qiao: 0000-0001-6351-455X 27



Aiqin Wang: 0000-0003-4552-0360 Xiaodong Wang: 0000-0002-8705-1278

Author Contributions The manuscript was written by the contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interest.

Funding Sources The National Natural Science Foundation of China (NNSFC 21573232, 21576251, 21676269, 21776269), National Key R&D Program of China (2016YFA0202-801), the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB17020100)

Acknowledgements This work was supported by the National Natural Science Foundation of China (NNSFC 21573232, 21576251, 21676269, 21776269), National Key R&D Program of China (2016YFA0202-801), the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB17020100).

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Reactivity of Methanol Steam Reforming on ZnPd Intermetallic Catalyst

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