ZrO2

Accepted Manuscript Effect of Ag loading on CO2-to-methanol hydrogenation over Ag/ CuO/ZrO2

Shohei Tada, Shigeo Satokawa PII: DOI: Reference:

S1566-7367(18)30191-2 doi:10.1016/j.catcom.2018.05.009 CATCOM 5403

To appear in:

Catalysis Communications

Received date: Revised date: Accepted date:

20 February 2018 13 May 2018 16 May 2018

Please cite this article as: Shohei Tada, Shigeo Satokawa , Effect of Ag loading on CO2-tomethanol hydrogenation over Ag/CuO/ZrO2. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Catcom(2017), doi:10.1016/j.catcom.2018.05.009

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Effect of Ag loading on CO2-to-methanol hydrogenation over Ag/CuO/ZrO2 Shohei Tada †* and Shigeo Satokawa * a

Department of Materials and Life Science, Faculty of Science and Technology, Seikei University,

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3-3-1 Kichijoji-kitamachi, Musashino-shi, Tokyo 180-8633, Japan.

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* Corresponding author

Shigeo Satokawa. E-mail: [email protected]

Present address : Department of Chemical System Engineering, Graduate School of Engineering,

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†

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Shohei Tada. E-mail: [email protected]

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The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan.

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Abstract The effect of Ag loading (wt%) of Ag/CuO/ZrO2 catalysts for the CO2 hydrogenation to methanol was investigated. The addition of a small amount of Ag (≤ 1 wt%) to CuO/ZrO2 did not change the

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turnover frequency of methanol production per exposed Cu sites (TOFmethanol) but increased the

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selectivity towards methanol due to a synergy developed between Cu and Ag, which was supported

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by H2-TPR studies. Keywords: Copper; Silver; Zirconia; Methanol Synthesis.

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1. Introduction

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The direct hydrogenation of CO2 into methanol has become a very active field of research

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because CO2 may be actively recycled, allowing mitigation of the greenhouse effect of CO 2 [1]. If

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methanol is synthesized from renewable hydrogen, this synthesis will contribute to the creation of a sustainable society. The problem of this methanol synthesis is the high cost of H2 production without

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using fossil fuels [2]. To address the matter, we have tried to develop new catalysts selective towards

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the CO2-to-methanol hydrogenation. During the exothermic CO2-to-methanol hydrogenation reaction (Eq. 1), the endothermic reverse water gas shift reaction (RWGS reaction, Eq. 2) will compete in particular at high temperatures [3]. Therefore, the catalysts are required to suppress the RWGS reaction. CO2 + 3H2 ⇄ CH3OH+ H2O

ΔrH (298K) =  49.5 kJ mol-1

CO2 + H2 ⇄ CO + H2O

ΔrH (298K) =

41.2 kJ mol-1

(1) (2)

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The specific catalysts which have been mainly investigated are Cu catalysts supported and/or promoted by ZnO [4-11] and ZrO2 [8-18]. We have investigated ZrO2-supported Cu catalysts and

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considered that ZrO2 has been particularly promising as a support, because it leads to a highly active,

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selective and stable catalyst [17-19]. In particular, we reported that Ag addition to CuO-ZrO2

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catalysts improved the selectivity towards methanol [18].We considered that the selectivity will be related to the formation of Cu-Ag alloy nanoparticles [18]. This phenomenon was also reported by

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Baiker et al. [20]. However, little attention has been given to the point concerning the likely synergy

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of the Cu-Ag catalytic system. In this study, we investigated the effect of Ag loading (wt%) on the

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catalytic performance and redox properties of Ag/CuO/ZrO2 catalysts in order to elucidate possible

2. Experimental

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2.1 Catalysts preparation

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relationship between the catalytic performance and Cu-Ag synergy.

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The Catalysis Society of Japan provided the amorphous ZrO2 (JRC-ZRO-5). First, a CuO/ZrO2 catalyst was prepared using the incipient wetness impregnation (IWI) method. Amorphous ZrO2 was impregnated with an aqueous solution of Cu(NO3)2 3H2O (Wako Pure Chemical Industries, Ltd.). Next, the obtained material was dried at 110 °C overnight and then calcined at 350 °C for 5 h. The material was named CZ350. After that, CZ350 was calcined again at 500 °C for 5 h. The thus obtained material was named CZ. The Cu loading of CZ was 10 wt%. In addition, a series of

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Ag/CuO/ZrO2 catalysts were prepared by the IWI method. CZ350 was impregnated with an aqueous solution of AgNO3 (Wako Pure Chemical Industries, Ltd.), dried at 110 °C overnight, and then calcined at 500 °C for 5 h. The obtained samples were named xACZ, where x is a Ag loading (0.5, 1,

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2, 3 and 5 wt%). As a reference, a 5 wt% Ag/ZrO2 catalyst was also prepared by the same IWI

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method. Amorphous ZrO2 was impregnated with an aqueous solution of AgNO3, dried at 110 °C

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overnight, and then calcined at 500 °C for 5 h. The obtained sample was named 5AZ. 2.2 Catalysts characterization

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The crystalline phase of catalysts was determined by powder X-ray diffraction (PXRD),

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equipped with Cu Kα radiation source, at a voltage of 40 kV and a current of 40 mA (Rigaku, Ultima

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IV). Temperature programmed reduction by H2 (H2-TPR) investigated reducibility of Cu and Ag

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species on the prepared catalysts in a flow system (MicrotracBEL, BELCAT-A). 50 mg of the samples were placed in a quartz tube and heated at 300 ºC for 1 h in Ar flow. Next, the samples were

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cooled down to 50 ºC in Ar flow. After that, the cell was purged with 4%H2/Ar. The temperature was

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then raised from 50 ºC to 500 ºC at a heating rate of 5 ºC min-1 under 4%H2/Ar flow (30 mL min-1). N2O titration was carried out according to the previous report [21]. About 500-900 mg of catalysts were placed in the quartz tube, connected to a flow system (MicrotracBEL, BELCAT-A) and treated at 300 °C for 30 min in 4%H2/Ar. The He gas was used as a carrier gas at 30 mL min-1, and the successive doses of 10wt%N2O/He gas were subsequently introduced into the He stream using a calibrated injection valve (27 μLN2O(STP) pulse-1) at 90 °C. A thermal conductive detector analyzed

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the amount of outlet N2O and N2. The number of accessible Cu surface atoms (CuS) was estimated according to Eq. 3. 2CuS + N2O → CuS2O + N2

(3)

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2.3 Evaluation of catalysts performance in direct hydrogenation of CO2 into methanol

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The CO2 hydrogenation was conducted in a fixed-bed tubular reactor at 10 bars (PID

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Eng&Tech, Microactivity Effi reactor). The reaction temperature was measured at the catalyst bed by a K-type thermocouple. After loading a catalyst powder (300 or 500 mg) and a quartz sand (5 g,

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Wako Pure Chemical Industries, Ltd.) into the reactor, the catalyst was treated under a flow of

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16%H2/N2 (72 mL min-1) at 300 ºC for 2 h under ambient pressure. After cooling to 270 ºC, a flow of

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CO2/H2/N2 (1/3/1, 14 mL min-1) was passed through the catalyst bed for 12 h at 10 bars, reaching a

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steady state. Subsequently, the catalyst bed was cooled down to 230 °C under the reaction conditions, and after 1 h the products were analyzed by an online gas chromatograph (Shimadzu, GC8A)

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equipped with FID (for methanol) and TCD (for N2, CO2, CO, CH4). In all experiments, CH4

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concentration in the outlet gas was almost zero. Thus, the carbon balance was estimated based on the concentrations of CO2, CO and methanol, using N2 as an internal standard. The reaction gas flow was changed from 28 to 70 mL (STP) min-1. Methanol selectivity, CO2 conversion, production rates of metahnol, CO and CO2, and turnover frequency of methanol per exposed Cu sites (TOFmethanol) were estimated using the following Eqs. 4-9. Methanol selectivity = Fmethanol,out / (Fmethanol,out + FCO,out)

(4)

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(5)

rmethanol = Fmethanol, out / W

(6)

rCO = FCO, out / W

(7)

rCO2 = rmethnol + rCO

(8) YN2O)

(9)

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TOFmethanol = rmethanol / (2

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CO2 conversion = (Fmethanol,out + FCO,out) / FCO2, in

where Fy, in is the total gas inlet flow rate of y species (mol h-1), Fy, out is the total gas outlet flow rate

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of y species (mol h-1), W is the catalyst amount used (gcat), and YN2O is the N2O consumption (μmol

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gcat-1) measured by N2O titration (see Section 2.2).

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3. Results and Discussion

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We examined the effect of Ag addition to CuO/ZrO2 catalysts on their catalytic performance for CO2-to-methanol hydrogenation. Figure 1a shows methanol and CO production rates for the

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as-prepared catalysts at 230 °C and 10 bars. The methanol and CO production rates over CZ were 1.2

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and 1.9 mmol h-1 gcat-1, respectively. After adding Ag to CZ, both of the rates were decreased. In particular, CO production rates over xACZ were dramatically decreased with increasing Ag loading. 5AZ produced less methanol and CO than the others. Table 1 shows the methanol selectivity against Ag loading for the xACZ catalysts. The selectivity was monotonically increased with increasing Ag loading. Of note, when the Ag loading was increased above 2 wt%, a change of the selectivity showed the different trend: the selectivity was almost the same (ca. 60%) despite gradually

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increasing. We calculated the apparent activation energy of CO2 conversion over CZ and 3ACZ from the slopes of the fitted lines in Arrhenius plots (Figure 1b). The rate data with CO2 conversion < 6% were used. The apparent activation energies for CZ and 3ACZ were found to be 71 and 58 kJ mol-1,

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respectively. In other words, Ag addition decreased the apparent activation energy by 13 kJ mol-1.

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This likely reflects differences in the nature of active sites between CZ and 3ACZ.

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Figure 1. (a) Methanol and CO production rates for the as-prepared catalysts at 230 °C. (b)

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Arrhenius plots of the CO2 reaction rate (denoted as rCO2) over CZ and 3ACZ. Reaction conditions: CO2/H2/N2 = 1/3/1; catalyst loading = 500 mg (a) or 300 mg (b); W/F = 430 gcat s L(STP)-1; pressure = 10 bar. Before the reaction test, all catalysts were reduced at 300 °C by 16% H 2/N2 for 2 h under ambient pressure. We assume a differential reaction condition with negligible heat and mass transfer effects because the conversions of the reactant (CO2 in this case) were kept below 10 % [22]. This matter was discussed in more detail in ESI.

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We investigated the effect of contact time on CO2-to-methanol hydrogenation over the prepared catalysts. Figure 2 shows the methanol selectivity at different CO2 conversions for the prepared

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catalysts. The CO2 conversion was increased by changing the contact time from 430 to 1070 gcat s

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L(STP)-1. According to our previous works [17, 18], the types of active sites should be reflected in

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the trend curves of methanol selectivity against CO2 conversion. Of note, for the xACZ catalysts, the trend curves appeared in an upper-right region compared to CZ and 5AZ catalysts. Therefore,

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addition of Ag to CZ is thought to create new active sites with high selectivity in CO2-to-methanol

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hydrogenation.

Figure 2. Methanol selectivity at different CO2 conversions for the prepared catalysts. The CO2 conversion was varied by changing the contact time from 430 to 1070 gcat s L(STP)-1. Reaction

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conditions: CO2/H2/N2 = 1/3/1; catalyst loading = 500 mg; reaction temperature = 230 °C; pressure = 10 bar. Before the reaction test, all catalysts were reduced at 300 °C by 16% H2/N2 for 2 h under

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ambient pressure.

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The natural question then arises: what was changed by adding Ag species to CZ? To shed some

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light on this, H2-TPR experiments were carried out to examine the reducibility of the samples (Fig. 3). Because the profile of 5AZ did not have any peak, Ag species in the as-prepared catalysts were

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not reduced by H2. Thus, we consider that Ag species was metallic after pretreatment at 300 °C in a

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He flow [18]. The other profiles (Fig. 3) possessed two peaks at 130 and 170 °C. The former peaks

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are attributed to the reduction of highly dispersed CuO [12, 23-27], and the latter peak can be

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assigned to the reduction of crystalline CuO particles [12, 23-26]. We estimated the mass ratio of highly dispersed CuO to crystalline CuO (Table 1), evaluated using the peak areas from the H2-TPR

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profiles. This ratio was found to decrease with increasing Ag amount, which means that Ag plays a

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role in the sintering of CuO [18].

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Figure 3. H2-TPR profiles of the as-prepared xACZ, 5AZ and CZ solid catalysts.

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Table 1 Characterization and catalytic performance results for the solids investigated. Ratio of CuO species [a] / % Highly

Methanol

[d]

selectivity [e] /%

[e]

/ h-1

dispersed Crystalline

CuO [b]

/ μmol -1 gcat 86

CuO [c]

0

48

52

0.5ACZ

0.5

37

63

1ACZ

1

31

69

2ACZ

2

30

70

3ACZ

3

15

85

5ACZ

5

21

79

5AZ

5

-

-

7.2

39

72

6.7

45

63

7.3

49

70

6.3

57

75

4.7

61

70

4.9

65

-

70

3

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CZ

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loading / wt%

N2O cons. TOFmethanol

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Ag

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Sample

[a] Estimated from the peak areas of H2-TPR (Figure 3); [b] Related with the peak at 130 °C; [c]

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Related with the peak at 170 °C; [d] Using spent catalysts; [e] Calculated form Figure 1a.

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N2O titration was carried out at 90 °C in order to estimate the amount of exposed Cu and Ag as

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summarized in Table 1. Almost no consumption of N2O was observed on 5AZ, indicating that N2O was inert to metallic Ag at 90 °C. The N2O consumption of 0.5ACZ (72 μmol gcat-1) was smaller than

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that of CZ (86 μmol gcat-1). The following two reasons for this difference are assumed. First, metallic

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Cu surface was covered with a subtle amount of Ag species, resulting in a decrease in the exposed Cu surface area. Second, Ag addition led to the particle growth of CuO. The latter assumption is in line with the result of H2-TPR (Fig. 3). The N2O consumption for xACZ remained unchanged when Ag loading increased from 0.5 to 5 wt%. Powder X-ray diffraction (PXRD) measurements of the as-prepared catalysts are shown in Figure 4a. The patterns for CZ and xACZ possessed several diffraction peaks attributed to t-ZrO2.

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The pattern of 5AZ had peaks of t-ZrO2 and m-ZrO2. The peaks of Cu species were not observed probably because the size of Cu species was smaller than the detection limit for PXRD (< 5 nm) or the Cu species was amorphous. When Ag loading was ≥ 3 wt%, a peak of metallic Ag was seen.

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PXRD patterns of spent catalysts are shown in Figure 4b. The information obtained about ZrO2 and

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Ag phases for the spent catalysts were similar to that for the as-prepared catalysts. Only the patterns

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of 3ACZ and 5ACZ had a peak of metallic Cu. In other words, when Ag loading in the xACZ catalysts was ≥ 3 wt%, xACZ possessed large Cu particles (> 5 nm), in line with the results of

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H2-TPR and N2O titration experiments. According to our previous work, xACZ can have the particles

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of Cu-Ag alloy [18]. In this study, however, we did not obtain any direct clue of the Cu-Ag alloy

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formation by using PXRD, probably because the amounts of Cu (10 wt%) and Ag (0-5 wt%) in CZ

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and xACZ were small. In addition, although we carried out XPS for CZ and 5ACZ (Figure S1 in

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ESI), it was very difficult to confirm the formation of Cu-Ag alloy.

Figure 4. PXRD patterns of (a) as-prepared and (b) spent catalysts.

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It has been reported that Cu-ZrO2 interfacial sites are highly active in CO2-to-methanol hydrogenation [17-19], while metallic Cu surface sites are active in CO production via the RWGS reaction [10, 11, 19]. Thus, we expected that all exposed Cu sites on Cu/ZrO2 catalysts are not

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equally active [28]. Furthermore, because Ag acted as a sintering aid for CuO, Ag species should

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interact with Cu species. Here, we calculated the turnover frequencies of methanol production per

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exposed Cu sites (TOFmethanol) for the CZ and xACZ catalysts, as summarized in Table 1, in order to evaluate the structure sensitivity and the interaction between Cu and Ag. TOFmethanol for CZ was

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found to be 7 h-1, whereas TOFmethanol for xACZ remained unchanged with increasing Ag loading

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from 0 to 1 wt%. When Ag loading of xACZ increased from 1 to 5 wt%, TOFmethanol for xACZ was

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gradually decreased to ca. 5 h-1. As described in ESI (Section S2), we expected that the particle

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growth of Cu species strongly affects the diminution of the interfacial sites (methanol production sites) compared to the decrease in the exposed Cu sites (CO production sites) [29-31]. In other words,

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CO2 hydrogenation would proceed mainly on the exposed Cu sites over the catalyst with large Cu

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particles, leading to low activity and selectivity in CO2-to-methanol hydrogenation. Because the particles of Cu grew and TOFmethanol decreased with increasing Ag loading, this hypothesis seemed to be in line with the experimental results. However, this hypothesis was inconsistent with the results in shown in Figure 2 and Table 1, where the selectivity towards methanol increased with increasing Ag loading. Next, we assumed the synergy between Cu and Ag. According to our previous work [18],

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Cu-Ag alloy can be formed on xACZ, although we did not obtain the evidence of Cu-Ag alloy formation by using PXRD. Of note, the trend of TOFmethanol against Ag loading for xACZ changed for the 1 wt% Ag loading. According to a reported phase diagram for a Cu-Ag system [32], the

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solubility limit of Ag into metallic Cu must be < 1 wt%. It was expected that the particles of Cu-Ag

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alloy were present on the surface of 0.5ACZ and 1ACZ, resulting in the formation of the interface

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between ZrO2 and Cu-Ag alloy, where the main active sites for CO2-to-methanol hydrogenation are found [18]. As shown in Figure 2, Ag addition to CZ led to the suppression of CO formation, thus to

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the enhancement of the selectivity towards methanol. When xACZ had the composition beyond the

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solubility limit of Ag (1 wt%), the particles of Cu-Ag alloy and metallic Ag can be present on xACZ.

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In fact, the peak of metallic Ag was observed for the 3ACZ and 5ACZ solids (Figure 4). If the

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metallic Ag particles block the interface between ZrO2 and Cu-Ag alloy, it seems reasonable that TOFmethanol fell with increasing Ag loading.

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It is to be pointed out that the TOF values reported in Table 1 would have been estimated in the

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accurate manner via the application of Steady State Isotopic Transient Kinetic Analysis (SSITKA) technique [33-36]. For instance, Mims et al. well calculated the steady-state rates of RWGS [33] and CO2-to-methanol hydrogenation [34] over Cu/SiO2. However, this subject was out of the present short communication. 4. Conclusions We examined the effect of Ag loading to CuO/ZrO2 catalysts (Ag loading: 0-5 wt%, Cu

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loading: 10 wt%) on the CO2-to-methanol hydrogenation. With increasing Ag loading from 0 to 1 wt%, the turnover frequency of methanol production per exposed Cu sites (TOFmethanol) remained unchanged and the selectivity towards methanol increased. In the case of further increasing Ag

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loading, TOFmethanol decreased. Although Ag acted as a sintering aid for Cu particles, the reduction of

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TOFmethanol and enhancement of the selectivity cannot be explained by the particle growth of Cu

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species. Therefore, the catalytic performance of Ag/CuO/ZrO2 were strongly affected by the synergy between Cu and Ag. The best mass ratio of Ag to Cu was found to be 0.1 in terms of high TOFmethanol

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and selectivity towards methanol values. We expected that the synergy is related to the formation of

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Cu-Ag alloy particles, and that selective synthesis of Cu-Ag alloy must play a key role for

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CO2-to-methanol hydrogenation.

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Acknowledgements

This work was supported by the Japan Society of the Promotion of Science (JSPS, NO. 15J10157). A

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part of this work was conducted at the Advanced Characterization Nanotechnology Platform of the

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University of Tokyo, supported by the “Nanotechnology Platform” of the Ministry of Education, Culture, Sports, Science and technology (MEXT), Japan.

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by transient isotopic techniques, Appl. Catal. B: Environ. 197 (2016) 168-183.

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GRAPHICAL ABSTRACT

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Highlights

We prepared Ag/CuO/ZrO2 catalysts for CO2-to-methanol hydrogenation.

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We investigated the effect of Ag loading on catalytic performances and properties.

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Ag addition to CuO/ZrO2 increased the selectivity towards methanol.

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The catalytic performance was related to the synergy between Cu and Ag.

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